The best time to visit Montenegro is April to October, when beaches, boat tours and hiking trails are all open. Within that window, June and September are the sweet spot for the Bay of Kotor and the Blue Cave: the sea is fully swimmable, days are sunny, and you avoid the July–August peak when Kotor's Old Town becomes one of the most congested spots on the Adriatic.
Kotor's climate is Mediterranean — mild, rainy winters and hot, sunny summers. Average August temperature is 24.9°C with highs often exceeding 33°C; January averages 7.5°C. Sea temperatures range from 14°C in winter to 25°C in August, holding around 23–24°C well into September. The town gets around 2,435 sunshine hours a year — more than Athens in a comparable latitude.
One local quirk: Kotor sits at the foot of the Vrmac Ridge, which means the town loses direct sun earlier than Tivat or Budva. In winter, the sun can drop behind the mountain as early as 2:00 PM — worth knowing if you're planning the fortress climb or Old Town photography outside summer.