Visas and passports are required by all foreign nationals except EU nationals who have a valid identity card. Visas are not required by British, Canadian, Australian, and American nationals who are visiting the country to sight see. People of these nationalities also do not require a return ticket upon arrival. However, if they plan to stay more than 90 days they need to apply for temporary residence permit a week before the 90-day deadline expires.
People from other nations are advised to contact their embassy to check on visa requirements. Visitors to Montenegro who are not staying at hotels are also required to contact and register with the police within 24 hours of arrival.
You can check information about visas at:
www.mip.gov.me/en/index.php/Visas-for-Foreign-Citizens/
Note: It is recommended that you check the latest visa and passport requirements with your embassy before planning a trip to Montenegro. These rules are subject to changes very often.
US citizens that need to renew, add pages, change name, or just get a new US passport, can Apply for a New US Passport here.