Also, this method would take the smell out of the mattress too.
All of the products that are needed for this cleaning hack are fairly cheap.
Shake the mixture together.

Image by Clean Freak & Germaphobe via YouTube
Then, spray the mixture on the pee spot and sprinkle baking soda over it.
Spread it around to form a paste mixture.
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Next, use fans to dry the mattress.

If its a small spot, you may use a hairdryer.
Also, take a dry towel and blot the area to get the pee out.
Youd need to get all of the liquid out of the mattress.

Continue to watchClean Freak & Germaphobeon YouTube for full instructions and details.
Overall, to avoid this problem, use a waterproof mattress cover protector.
Sometimes the stains can come from excess sweating, from a toddler or dog.
I have 2 layers of protection on my mattress because I have a protector and a cooling sheet.
This is an easy method to try because her instructions are thorough.