Can I pray in a locker room or changing room?

Yes, usually, if you pick the right spot. The changing area is acceptable if the floor is clean and dry, nobody is uncovered in front of you and you're away from the showers and toilets. Many scholars dislike praying inside bathing areas, so choose the cleanest spot, not just the nearest one.

The whole earth is a place of prayer

The Prophet ﷺ said that the earth has been made a place of prayer for his followers, so they can pray wherever the time of prayer comes (Sahih al-Bukhari 335). You don't need a mosque or a prayer room. So the starting point is that a locker room can be fine, provided it's clean and suitable.

Why some scholars are careful

There is a report that says the whole earth is a place of prayer except public baths and graveyards (Sunan Abi Dawud 492). Scholars discuss how strong this report is, and how far it applies. Some, including many Hanbalis, avoid praying in bath areas completely. Others say it is only disliked. The reasons behind the caution are practical: wet floors, possible impurities, uncovered people and distraction.

That's why praying inside a shower stall or a toilet cubicle is a bad idea, while the dry changing area with benches is much better. See Can I pray in a room with a toilet?

How to pick the cleanest spot

Stay dry. Choose a corner of the changing area, away from the shower floor and the toilet doors.
Bring a mat or towel. A clean towel or small travel mat marks out a clean spot, so you don't need to worry about the floor.
Check for uncovered people. Don't pray facing or beside someone who is undressing. Wait a minute or go elsewhere. See Can Muslim men change in an open gym locker room?
Make wudu at the sinks, and dry your feet so you don't slip.
Face the qibla, the direction of the Kaaba in Makkah, and keep it simple and quick.
Lock up your things so you're not worried about them while praying.

Better options at most gyms

Before you use the locker room, look for a quieter place that is easier to keep clean:

An empty studio between classes. Ask reception which rooms are free.
The stretching area, early or late in the day.
A quiet corridor or stairwell.
Your car or a clean patch of grass outside.

See Where can I pray at the gym?

If the locker room is your only choice

Don't miss the prayer time because the place isn't perfect. A clean, dry spot on a mat in the changing area is acceptable for most scholars, and praying there is far better than delaying the prayer. If the same situation comes up again and again, ask the gym if you may use a spare room. Many will say yes.

Key points

A changing area can be fine if it is clean, dry and not crowded.
Avoid praying inside showers or toilet cubicles.
Some scholars dislike praying in bath areas, so look for a better room first.
Don't delay a prayer because the spot isn't ideal.

Tip: Ask reception once if there is a quiet room you can use for five minutes. Staff often remember you, and you won't need the locker room again.