The 40-year-old former Netherlands international joined United from Fulham in 2005 and has made 245 appearances for the Old Trafford club, winning three Premier League titles and the 2008 Champions League crown. Van der Sar's wife fell seriously ill in December 2009.
"It is now time to pay attention to my family," he said on the website of his management company.
"I cannot really identify a time when it (the decision) happened.
"Let's just say that it was playing on my mind from the moment Annemarie had her stroke.
"She has fought back from it. We decided on another year in England and thus to stay at Manchester United.
"But, once engaged in the season, the thought of saying goodbye started to gnaw a bit more emphatically."
Van der Sar, who played a record 130 times for his country, began his career at Ajax Amsterdam before moving to Juventus and on to Fulham in 2001
He set an English league record in 2009 for time without conceding a goal when he went more than 21 hours unbeaten.
United are five points clear at the top of the Premier League and Van der Sar has played in 21 of their 23 league games this season. "It is a difficult process" he said. "After a defeat, I thought differently than after playing a few good games in a row.
"My age played no role,I am 40 years old, but I still feel fit.
"And then the decision came suddenly. Do not ask me how or why, but suddenly you know. That was sufficient."
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