When it comes to swimwear underboob, you will find two very different looks.
In the first, the swimsuit is designed with an opening below the bust. The cutout is part of the pattern, just like a keyhole, open back, or side cutout.
In the second, breast tissue is slipping below the cup of a normal bikini top that was never meant to show it.
Those situations may look similar in a photo, but they mean very different things for fit. This guide is about to share further details with you. Let's sink in.
When Do We Call A Bikini Underboob?
An underboob bikini is a swim style designed to show part of the lower breast, usually through a shaped opening, raised cup edge, cutout, or open section below the bust.
The idea is sometimes described as “reverse cleavage”: instead of showing more skin above or between the breasts, the design creates an opening underneath them.
Bustle documented the look in its underboob-bikini guide, updated in 2024, including bikinis and one-pieces built with deliberate lower-bust cutouts. Source
Compare Designed Underboob & Accidental Underboob
| What you see |
Designed underboob |
Possible fit problem |
| Lower-bust opening |
Built into the pattern or cutout |
Breast tissue has slipped below a normal cup |
| Shape |
Opening usually looks deliberate and balanced |
Exposure may be uneven or increase as you move |
| Band |
Should still stay in its intended position |
May ride upward or fail to anchor the top |
| Cups |
Designed around the cutout |
May be too small, shallow, or badly positioned |
| Movement |
The opening stays fairly predictable |
Coverage keeps changing or needs adjustment |
| Goal |
Fashion and intentional skin exposure |
Usually not the look the shopper chose |
What Makes Intentional Swimwear Underboob Look Controlled?
The best underboob designs do not simply remove fabric. They give the remaining fabric a clear job.
1. The opening has a defined shape
Look for a cutout with clear edges rather than a cup that simply ends too early. A designed opening makes the amount and position of exposed skin easier to predict.
2. The top still has an anchor
Depending on the style, this can come from a back band, side structure, upper cup, straps, or the body of a one-piece. An underboob opening should not mean the whole garment is free to move.
3. The straps can be adjusted where needed
Adjustable straps can help position the upper part of the top. But they should not have to solve every fit problem. If tightening the straps is the only thing stopping the top from moving, look at the rest of the construction too.
4. The cutout stays in the same area when you move
Underboob is meant to reveal skin. The key question is whether it reveals roughly the amount you chose rather than becoming much more open when you raise an arm, bend, or walk.
5 Underboob and Cutout Bikini Tops to Compare
Not every cutout bikini creates the same amount of underboob. That is useful rather than confusing: you can choose how literal you want the trend to be.
The first and last styles below make the lower-bust opening a clear part of the design. The middle three use keyholes, center cutouts, or structured openings to give a similar skin-revealing effect with different levels of containment.
BERLOOK Yellow Textured Underboob Cutout Bikini Top
$39 · XS–XL · 3/4 cup · Bralette · Purpose-built underboob cutout
Best for the clearest version of the underboob trend. The lower-bust opening is part of the pattern rather than something created by wearing a normal cup too small. The textured bralette uses removable soft cups and high-stretch recycled-polyester fabric. Choose this when the exposed lower-bust line is the look you actually want, but remember that padding adds shape rather than guaranteeing strong support.
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BERLOOK Dark Green Textured Cutout Bikini Top
$28 · XS–XL · 3/4 cup · Bralette · X-cross back · High stretch
Best for trying the cutout look without committing to a large underboob opening. Its circular center cutout exposes skin between and below the cups while the surrounding 3/4-cup bralette keeps more fabric around the bust. The X-cross back also gives a different tension path from a simple triangle bikini. Think of this as an underboob-adjacent option rather than a true open-bottom cup.
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BERLOOK ECONYL® Black Cut Out Bikini Top
$55 · XS–XL · 3/4 cup · Plunging neckline · Criss-cross tie · ECONYL®
Best for a cleaner black cutout with more fit adjustment. The lower-center opening gives a controlled flash of skin while the strappy back and drawstring construction allow more adjustment than a fixed bikini frame. The shell uses 85% ECONYL® regenerated nylon. Fit deserves extra attention if you are fuller on top: most current reviewers report true-to-size experiences, but one larger-bust reviewer found this top small and returned it.
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BERLOOK Green Geometric Textured Cut Out Bikini Top
$35 · XS–XL · 3/4 cup · U-neck · Keyhole cutout · Criss-cross tie back
Best for a more contained take on skin-baring swimwear. Instead of opening the whole lower cup, this style concentrates the reveal in a center keyhole and keeps more fabric beneath the bust. The criss-cross tie back lets you fine-tune the body fit. Its current review pool is larger than the other BERLOOK cutout examples here, but feedback is still mixed enough that the size chart matters: at least one reviewer reports the style running small.
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IVY Berlin Chained Up Bikini Top
€54 · XS–XL · Explicit underboob cutout · Chain detail · Handmade in Berlin
Best for making underboob the main statement. IVY Berlin explicitly describes this design as an underboob cutout and pairs the opening with chain hardware for a much more festival-led look. It is useful in this comparison because there is no ambiguity about whether the lower-bust exposure is intentional. The product page positions it strongly around statement and festival dressing, so we would not assume technical swim support or rough-water performance without testing it yourself.
View at IVY Berlin
Our takeaway: if you specifically searched for swimwear underboob, start by deciding how much of the effect you actually want. The Yellow Textured Underboob top and IVY Chained Up make lower-bust exposure the point of the design. The other BERLOOK options keep the cutout more centralized, which can give you the visual idea of the trend without removing as much fabric beneath the bust.
Whichever version you choose, do not use a smaller size as a shortcut to create more underboob. The cutout should come from the pattern; the size should still come from your measurements.
The Relationship between Underboob Bikini and Low Support
Coverage and support are related, but they are not the same thing.
An underboob top can expose skin while still using a stable band, structured upper cup, side support, adjustable straps, or even an underwire placed around the opening. On the other hand, a full-coverage top can still feel unsupportive if the band is loose or the cups do not fit.
The trade-off is that removing fabric can reduce containment. If the opening is large and the top has very little surrounding structure, it may be better suited to lounging or styling than to fast swimming or rough water.
So instead of asking only, “Is underboob supportive?” ask:
- How stable is the band or base of the top?
- How much breast tissue is held by the remaining cup?
- Can the straps be adjusted?
- Does the opening stay where it started when you move?
- What activity was the product actually designed for?
The 5-Step Underboob Bikini Fit Test
1. Put the band in its natural position
Do not start by pulling the straps as tight as possible. First make sure the body of the top sits where the design intends it to sit.
2. Check the opening on both sides
Human bodies are not perfectly symmetrical, so the two sides do not have to look mathematically identical. But a big difference can be a sign that the cups or straps need adjustment.
3. Raise both arms
Watch whether the entire top moves upward or whether the underboob opening becomes much larger than it was while standing still.
4. Bend and twist
The top should remain secure enough for your comfort level. If you immediately need to reposition it, it may not suit the activity you have in mind.
5. Test your real use
A bikini for a pool lounger does not need to pass the same test as a bikini for surfing. Mimic the movements you will actually make.
Underboob vs. Center Cutout, Not Always the Same
Another source of confusion is the word “cutout.”
A center-front keyhole may show skin between the breasts while keeping the lower breast covered. A bandeau may have an opening below the center only. A one-piece may have a large torso cutout that reaches the lower bust.
These can all create a similar visual mood without producing the same amount of underboob.
When shopping online, use the product photos together with words such as:
- underboob cutout;
- bust cutout;
- keyhole;
- cut-out underwire;
- peekaboo;
- open bust;
- and center-front cutout.
Then check the front and side views instead of assuming those labels all mean the same design.
Check These Before Buying Swimwear Online
-
Does the product actually call out a cutout? If not, the model may simply be styling the top differently.
-
Are there side photos? They show where the opening ends and how the cup sits.
-
Are bust or model measurements listed? These give more context than the letter size alone.
-
Can the straps or back adjust? This helps fine-tune position.
-
How much cup coverage is provided? Demi and 3/4 cups create different levels of containment.
-
Is the style meant for swimming or mainly styling? Do not assume every product called a “bikini” has been tested for sport.
-
What do the reviews actually say? Comments about fit, slipping, cup room, and movement are more useful than “so cute.”
You May Also Ask
Is underboob a sign that my bikini is too small?
It depends on the design. If the top was made with an underboob cutout, visible lower breast is intentional. If a normal full or supportive cup is supposed to contain the breast and tissue is slipping underneath it, the size or cup shape may be wrong.
Can an underboob bikini still be supportive?
Yes, depending on its structure. A stable band, supportive upper cup, adjustable straps, side structure, or underwire can still contribute support. But a very open cutout usually provides less containment than a more covered design.
Are underboob bikinis only for small breasts?
No. The more useful issue is whether the product has enough room and stability for your bust. Fuller-bust shoppers may benefit from firmer underbust construction, adjustable straps, or more structured cups.
Is an underboob cutout the same as a keyhole bikini?
Not always. A keyhole usually creates an opening in the center of the top. It may reveal cleavage or lower-bust skin depending on where it is placed. “Underboob” refers specifically to visible skin beneath the breast.
Warpping up
The useful way to understand swimwear underboob is not to ask whether showing the lower bust is “right” or “wrong.”
Ask whether it is intentional.
A well-designed underboob bikini uses the opening as part of the garment. A badly fitting standard bikini exposes breast tissue because the cup, band, or size is not doing what it was designed to do.
That difference changes how you should shop.
If you want the look, choose a purpose-built cutout such as BERLOOK's Yellow Textured Underboob Cutout Bikini Top rather than forcing a smaller conventional cup to create it. Then test the band, cup position, straps, and movement just as carefully as you would with any other swimsuit.
Because the most successful underboob look is not simply the one that shows skin. It is the one that shows exactly the amount you meant to show—and still feels like your swimsuit when you start moving.
Research note: Product pages and availability were checked in August 2026. Fit guidance below draws on current specialist swimwear guidance and peer-reviewed bra-fit research. Bra studies are useful for understanding bands, cups, and straps, but they are not direct tests of underboob bikinis, so we do not treat them as such.
