The Trident's management said that about 100 of the hotel's 550 rooms had been booked for Sunday, while all four of its restaurants were open.
"It is a statement to terrorists that this does not close down business. [Attacks do] not close down hotels," Rick McElrea, a Canadian who lives in Mumbai, said as he visited one of the restaurants for breakfast.
"I don't feel any fear. I feel hope. The terrorists failed and Mumbaikars won."
Around 10 armed men stormed the Trident-Oberoi, the Taj Mahal Palace and other Mumbai landmarks on November 26.
The men, alleged to be linked to Pakistan-based group Lashkar-e-Taiba, battled Indian security forces for three days, leaving the two hotels seriously damaged.
Indian intelligence and security services have been criticised for failing to prevent the attacks.
New Delhi is calling on Islamabad to take stronger action against those it suspects of planning the raids.
Pakistan has cracked down on a Lashkar-linked charity but has declined to extradite any of the arrested suspects, saying India must share intelligence implicating the group.
The re-opening of the attacked hotels comes a day after Ronald Noble, the head of Interpol, the international police organisation, met P Chidambaram, India's minister of home affairs, to discuss co-ordinating an investigation into the attacks.
Interpol has said it is willing to distribute suspects' names, fingerprints, DNA profiles and photographs to police agencies around the world.
Washington has also offered its assistance into the investigation.
Admiral Timothy Keating, the commander of the US Pacific command, said India had delivered a "calm, measured response" to the Mumbai assault, comparing it to attacks in the US on September 11, 2001.
"We are working through the initial parts of a package that ... we would offer to India to help them understand some of the lessons learned that we very painfully learned in the wake of our 11 September attacks - in information sharing, collaboration and co-operation," he said.