Upvote:5
Let's ask Timatic:
Passport required.
- Passports and other documents accepted for entry must be valid for the period of intended stay.
Visa required, except for Nationals of USA for a maximum stay of 90 days.
- This does not apply to passengers with an emergency or a temporary passport.
A max. stay of 90 days is granted within any 180 days period to nationals of USA.
Warning: Visitors not holding return/onward ticket could be refused entry.
The US Department of State has further information for visitors to Ukraine. Note that it says proof of health insurance and sufficient funds are required.
There's important information and warnings in there should your travel plans involve Crimea or Eastern Ukraine.
Have a nice trip.