Lopez, 33, and her former beau, who also goes by Puff Daddy and whose real name is Sean Combs, 34, were seen at the Opium Garden Saturday night, London's Sun reported Tuesday.
"They were really into each other," one onlooker said. "They were laughing and joking and having a wild time. It's like they had never split up."
The couple was engaged in 2000.
While Lopez partied in Miami, Affleck was in London promoting his new film "Paycheck."
Affleck, 31, told London's Channel 4 that he and Lopez still have not set a wedding date, adding that the "escalating frenzy" caused them to postpone last September's nuptials.