"I have sent solidarity regards to Julian Assange from the left in Germany and the online community in Germany," said Dagdelen in a statement issued after the visit.
"We, as peace- and freedom-loving people in Germany and around the world, are greatly indebted to Mr. Assange.
"He helped to uncover the war crimes in Iraq and in Afghanistan. He showed us how dirty and bloody these wars were and are, and how much we have been fooled by our governments."
Dagdelen, a member of the socialist Left party who sits on the foreign affairs committee in the lower house, wants to help find a solution to the diplomatic standoff around Assange's asylum request.
"Mr Assange was very pleased about my visit. I am amazed that I am the first MP to visit him," she added.
"I would like more of my colleagues from other countries to come over to London and seek a humanitarian solution to this crisis. We as parliamentarians should demand that our governments act.
"Unfortunately, the German government has taken no action to find a solution. I have informed Mr Assange about this deplorable fact."
Britain has said that Assange will be arrested if he sets foot outside the embassy and extradited to Sweden, where he is wanted for questioning over alleged sex crimes under the terms of a European arrest warrant.
WikiLeaks angered Washington by releasing tens of thousands of classified files about the wars in Iraq and Afghanistan, as well as US diplomatic cables which gave often unflattering views of world leaders.
Assange has said he fears that if he is forced to go to Sweden he will eventually be extradited to the United States for prosecution over the leaking of classified US documents.
Dagdelen added: "Extraditing Mr Assange to Sweden, which would probably involve him being taken into custody, would trigger a chain of events that would hamper pre-emptive measures to stop him being extradited to other countries, specifically the US, where Mr Assange faces long-term imprisonment or possibly even the death penalty.
"Under international law, if someone faces extradition to a country where they are in danger of being sentenced to death for their political work, there is no question that this constitutes undisputed grounds for granting refugee status."
The German lawmaker added that she would seek meetings with British and Swedish diplomats in Berlin, in an attempt to find a solution to the standoff.
Ecuador, which has granted Assange diplomatic asylum, was angered by what it saw as Britain's threat to enter the embassy and arrest Assange.
London insisted it never threatened to enter the building and merely made Ecuador aware of the existence of a law which would allow it to do so.