What should I wear to the mosque?

Yes, there are a few simple rules: wear something clean, modest and comfortable. Men should cover at least from the navel to the knees, and women should cover everything except the face and hands. Loose clothes you can bow and sit in, clean socks and a fresh smell are all you really need. You don't have to wear traditional or "Arab" clothing.

Dress well for Allah's house

The Quran tells people to take their adornment at every masjid. Scholars explain this as covering properly and wearing decent, clean clothes for prayer. You don't need anything expensive: a clean pair of trousers and a shirt or sweater is fine. See Can I pray in jeans?

۞ يَـٰبَنِىٓ ءَادَمَ خُذُوا۟ زِينَتَكُمْ عِندَ كُلِّ مَسْجِدٍ "O children of Adam, take your adornment [i.e., wear your clothing] at every masjid..." Quran 7:31

What men should wear

Cover navel to knee: that's the minimum, but most scholars recommend covering the shoulders too in prayer.
Avoid shorts and tight gym wear: when you bow and prostrate, short or tight clothes can ride up or show too much.
Skip offensive slogans or images on T-shirts.
Perfume is welcome: the Prophet ﷺ encouraged men to bathe and use perfume before Friday prayer (Sahih al-Bukhari 883).

What women should wear

Cover the hair and body: loose, non-see-through clothing that covers everything except the face and hands. The Hanafi school also allows the feet to show in prayer, while others ask you to cover them.
Long sleeves and a long skirt or loose trousers work well. Many mosques keep spare scarves and prayer garments.
Go light on scent: scholars discuss perfume for women in mixed public spaces, so many keep it minimal. See Can Muslim women wear perfume in public?

If you're still working out what covering means for you, see What does hijab actually cover?

Shoes, socks and smells

You take your shoes off at the entrance, so wear clean socks without holes. Avoid strong food smells: the Prophet ﷺ said whoever has eaten garlic shouldn't come to the mosque (Sahih al-Bukhari 853), and he included onions and leeks, adding that the angels are bothered by what bothers people (Sahih Muslim 564). The same idea applies to cigarette smoke and sweaty gym clothes. If you come straight from work, a quick wash and deodorant go a long way.

If you're not Muslim and visiting

Visitors are welcome. Dress modestly: long trousers or a long skirt, covered shoulders and arms. Women are usually asked to cover their hair in the prayer hall, and many mosques lend a scarf at the door. See Can I visit a mosque if I'm not Muslim?

Key points

Clean, modest and loose clothing is the goal.
Men cover at least navel to knee; women cover all but face and hands.
Wear clean socks, since shoes come off.
Avoid garlic, onion and smoke smells before going.

Tip: Keep a "mosque kit" in your bag or car: a spare pair of socks, a small deodorant, mints and, for women, a light scarf. Then you're ready whenever you pass a mosque at prayer time.