The Vanarama National League South side parted company with previous boss Paul Barnes earlier this week after a run of five straight defeats.
The Hamlet's last win came in mid-January, picking up just one point in their ten games that followed.
New boss Hayrettin was the South Manager of the Season last year after guiding Maidstone to the title.
He's also had spells at Braintree and Luton Town - where he won the National League title as part of John Still's backroom team.
His new side sit two points above the bottom four - but he says he can't wait to get started against St Albans City tomorrow.
"I am excited to be at Dulwich Hamlet," he said.
"The project here is incredibly appealing. We have a committed and passionate fanbase, who have a clear expectation for the club.
"First and foremost though, my immediate focus is this weekend."
Hamlet chairman Ben Clasper added: “Hakan has great pedigree at this level, with a track record of success, and with the club in its current position and its aspirations for how we feel we should perform at this level, Hakan is best placed to deliver on both fronts."