He gained 72% of the vote, the election commission announced, against 25% for his closest challenger Bobi Wine, who has condemned what he described as "fake results" and "ballot stuffing". He has not provided any evidence and the authorities have not responded to his allegations. BBC reported.
Wine has called on Ugandans to hold non-violent protests.
Museveni, 81, first came to power as a rebel leader in 1986 but since then has won seven elections.
The election process was marred by violence and Wine, a 43-year-old former pop star, says that at least 21 people have been killed around the country in recent days.
The authorities have so far confirmed seven deaths.
Access to the internet has been cut in the country since Tuesday, making it hard to verify information.
The authorities say the blackout was necessary to prevent misinformation, fraud and the incitement of violence - a move condemned by the UN human rights office as "deeply worrying".
Wine has demanded that the internet be restored.
