An executive close to the network said management at parent NBC Universal wants to accelerate changes at the third-place cable news channel, but added the decision behind Mr. Kaplan's departure was made by mutual consent.
Staff members were informed of the departure this afternoon, and a note from NBC News President Steve Capus informed department heads they would be reporting to him until a new head is named.
"Rick has been a tireless champion for the network and all the hard work you do each and every day," read Mr. Capus' note, which was obtained by Advertising Age. "He has led MSNBC through a period of impressive growth especially in primetime. MSNBC continues to be an enormously important asset in the NBC News portfolio."
In a separate note to staff, Mr. Kaplan wrote: "I want to let you all know that today I'm leaving MSNBC. It is not often in professional life that someone has the opportunity to end his tenure on such a high note. I couldn't be more proud of the progress we've made together over the last two and a half years. My genuine hope is that you continue to grow and build on the momentum we've achieved together at MSNBC."
Mr. Kaplan's contract was said to be expiring at the end of this year, but that could not immediately be confirmed. Mr. Kaplan declined to comment.
Before joining MSNBC, Mr. Kaplan oversaw all of ABC's hard-news programming. Priot to that, Mr. Kaplan was a teaching fellow at the Shorenstein Center of Harvard University's John F. Kennedy School of Communications.