What you wear under your costume will vary depending on the type of costume you’re wearing and what makes you feel most comfortable. This means you have to take your body type and the weather into account before you can make the right choices. You’ll also want to use specific undergarments for bodysuits, crossplay, and armor costumes to make sure you look and feel your best.
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Tights
If you’re wearing a skirt or a dress, or even shorts, tights can be a big benefit to your costume. Wearing a pair of nude-colored tights underneath your cosplay can help to even out your skin tone, prevent chafing, and keep knee-high socks from falling down. You can even sew or pin the socks to your tights for added holding ability.
Instead of cheap tights, I recommend using a pair of dance tights. These are sturdier, less see-through, and generally last longer than other types of tights or pantyhose. These kinds of tights are usually available in various nude skin tones as well.
The other good reason for wearing tights is if you are wearing a very short skirt or a leotard for your costume. They will help to keep everything covered so you don’t have to worry when going up stairs or sitting down. You can also choose to wear a pair of shorts under your skirt or dress to help.
Shapewear and supportive clothing
If you have any kind of tight or form-fitting costume, it’s useful to wear some kind of shapewear underneath. Even if you are very athletic, shapewear can be used to smooth out the surface of our skin to prevent wrinkles throughout the day. Personally, I find that the shapewear also acts as a reminder throughout the day to maintain good posture so I look better walking around.
In hot months, shapewear will also aid in absorbing moisture from your skin. It can help to prevent sweat spots on your costumes and keep you looking fresh and poised all day long.

Body tape
Body tape is super useful for items of clothing that work on a cartoon character, but don’t work as well with real-life physics. You can use this to keep your chest covered in revealing outfits or prevent a wedgie in a bathing suit costume. If you don’t want to wear tights, you can use body tape to hold your socks up.
You can also use body tape to attach armor pieces directly to your skin or keep the edges of a cloak exactly where you want them to be. If you’re running a little late on completing your costume, you can also use body tape to help hold everything together.
Dress for the weather
When deciding what to wear under your costume, you also need to take the weather into account. If you’re going to a winter cosplay event, it’s a good idea to add layers underneath the costume, especially if you will be wearing a thin bodysuit. You can get warm materials that aren’t too bulky (like compression shirts and leggings) so that they won’t detract from your overall costume.
In the summer, you want to focus more on preventing sweat spots. Wear an undershirt or tank top under your cosplay to help soak up any sweat before it gets to the costume. It may seem odd, but one thing you can do to prevent armpit sweat is to stick pantyliners within the underarm of your costume pieces. These are very absorbent and will soak up sweat, leaving your costume nice and dry.

Undergarments for crossplayers
For women cosplaying as male characters, you will typically want to flatten your chest a bit. Do NOT use an ace bandage or stiff fabric option. Your chest needs room to expand so that you can breathe properly. When worn incorrectly for long periods of time, ace bandages can cause damage to your chest (potentially even causing cracked ribs) and will cause difficulty in breathing.
Instead, you should use a chest binder. I recommend the brand Underworks. They are known for having binders that are effective even for people with large chests, but also expand enough to make them comfortable and breathable.
For male crossplayers, you will want to look for push-up bras to help create the curves you’re looking for. Depending on the type of costume you are wearing, you also might want to learn about dance belts and compression shorts, to help everything stay smooth down there.

What to wear under a superhero or bodysuit
Most superhero suits are made of highly stretchy spandex. It’s an incredibly useful material, but unfortunately, it tends to show every lump and bump underneath it. In most cases, you do not want to wear a spandex bodysuit directly over your skin with nothing in-between.
For us ladies, wearing spandex suits is a little easier than for men. Typically all you will need to do is wear some kind of shapewear or compression clothing underneath. You probably also want to wear pasties to prevent your headlights from showing.
For men, compression clothing and shapewear are also useful. However, you’ll have to do a little more to prevent your bulge from being a little NSFW. The standard advice is to use a dance belt to keep everything smooth.
Other than the basics, I have very little experience with the intricacies of male underwear with spandex. There is a much more thorough article about this topic on Pop Culture HQ that you can check out.
What to wear under armor costumes
Armor costumes are usually not as complicated as spandex. If the character has a shirt under the armor, you can easily attach the armor directly to the shirt and wear your usual underwear. It gets a little more complicated when the character appears to have armor placed directly onto their skin or has bra/underwear/bathing suit armor.
In these cases, you’ll want to use well-fitting and sturdy underwear and bra. Design your armor directly onto these articles of clothing. When wearing the armor, you’ll want to wear a second, smaller pair of underwear beneath the armor and pasties to prevent any unfortunate accidents.
You can also use body tape to more securely keep your articles of armor directly against your skin to prevent any kind of visible attachments that will detract from the overall look of the cosplay.