Grindel was given the item by Ukrainian businessman and Uefa vice-president Grigoriy Surkis.
He said: "The price of the watch was 6,000 euros (£5,152). I did not know the brand or its value at the time.
"I apologise for my less-than-exemplary action of accepting the watch."
Grindel insisted there had been no conflict of interest in accepting the watch, adding: "Mr Surkis had no financial interest in the DFB. He never asked me for any support, then or after.
"It was clear at the time that he would not run again for Uefa's Exco, to which he no longer belongs."
Grindel's departure ends a controversial three-year spell in charge of the DFB.
A number of his actions in the role, including introducing Monday night games, late kick-off times, and a ban on pyrotechnics, proved unpopular with fans while last week he was accused of failing to declare additional earnings in an investigation by Der Spiegel.
The German magazine identified a 78,000 euros (£66,986) income he had received from a DFB subsidiary that had not been made public.
The accusation was rejected by the DFB, which said Grindel took on the position with its subsidiary company only after he became president, and so was not obliged to declare the earnings at the time.
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