He was welcomed by a large crowd of supporters as he went to collect nomination papers from the party's offices in Kampala, the capital.
He said in a speech to his followers that, if reelected, one of his goals is to fight official corruption.
“In this time I want us really to wipe out corruption,” he said.
Museveni first took power as head of a rebel force in 1986. He has since been elected six times, though recent elections have been marred by violence and allegations of vote rigging. His main opponent in the last election was the popular entertainer known as Bobi Wine, who has also declared his candidacy in the polls set for January.
Wine, whose real name is Kyagulanyi Ssentamu, has seen many associates jailed or go into hiding as security forces cracked down on opposition supporters.
Museveni has dismissed Wine as “an agent of foreign interests” who can't be trusted with power. Wine has been arrested many times on various charges, but has never been convicted. He insists he's running a nonviolent campaign.
Decades ago, Museveni himself had criticized African leaders who overstayed their welcome in office. In Uganda, lawmakers did the same thing for him when they jettisoned the last constitutional obstacle — age limits — for a possible life presidency. His son, army chief Muhoozi Kainerugaba, has asserted his wish to succeed his father, raising fears of hereditary rule.
A long-time opposition leader, Kizza Besigye, has been jailed since November over alleged treason charges his lawyers say are politically motivated. Besigye, a physician who retired from Uganda's military at the rank of colonel, is a former president of the Forum for Democratic Change party, for many years Uganda's most prominent opposition group.
Uganda has never seen a peaceful transfer of power since independence from Britain in 1962.