No, mens' bathroom is wrong. It's men's bathroom. The placement of apostrophes is explained here. Here's a 100% rule: (1) Take your word (regardless of its ending or its number, i.e. singular or plural). (2) Add es (3) Replace the e with an apostrophe. (4) If you're left with an s's ending, delete the last s. men > menes > men's cat > cates > cat's cats > catses > cats's > cats' people > peoplees > people's ladies > ladieses > ladies's > ladies' (This works for everything because the possessive form used to be shown by adding es to everything.)