British police remained stationed outside the building, where Assange is holed up, unable to cross the threshold onto Ecuadorean territory, as the American pop star walked up the stairs and spent several hours inside.
Lady Gaga, 26, was allowed through the front door of the embassy, 'The Times' reported.
Gaga, who was at Harrods to launch her latest perfume on Sunday, returned to Knightsbridge last night where she spent approximately five hours at the embassy, which is located directly behind the department store.
The pair are believed to have had dinner together, and then posed for a grainy photo which was posted on the Twitter feed of 'LittleMonsters.Com', Gaga's fan site.
The visit came after the British rapper M.I.A tweeted a message to Gaga on Sunday, inviting her to visit Assange while she was in London.
"If ur at harrods today, come visit Assange at the Ecuador embassy across the street im there. Ill bring TEA and CAKE," M.I.A. Tweeted Gaga.
Gaga was photographed leaving the embassy shortly after midnight, however it is not known if M.I.A had also visited Assange.
It is not known how the pop singer knows the 41-year-old Australian, however there is a connection between Gaga and his WikiLeaks website.
Bradley Manning, the US army intelligence analyst alleged to have leaked cables to WikiLeaks, reportedly boasted about how he stole the data, by taking a Lady Gaga CD into the office, wiping the music and downloading files while pretending to lipsynch to her songs.
In November, 2010, it was reported that Manning allegedly told a fellow hacker in a chatlog: "I would come in with music on a CD-RW labelled with something like 'Lady Gaga'...Erase the music...Then write a compressed split file. No one suspected a thing ...Listened and lip-synched to Lady Gaga's Telephone while exfiltrating possibly the largest data spillage in American history."
Assange has been holed up inside the embassy since he sought refuge there on June 19 to avoid extradition to Sweden, where he is wanted for questioning about allegations of rape and sexual assault.
Assange, who has denied the allegations, fears he could be extradited to the US from Sweden to stand trial for espionage, on account of the trove of leaked American diplomatic cables and military logs published on his whistleblower website.
