The 33-year-old son of Kremlin spokesman Dmitry Peskov says he served with the Wagner mercenary group in Ukraine for nearly six months.
Edneüs reports citing BBC that Nikolai Peskov said "it was my duty... I couldn't sit to one side watching as friends and others went off there."
Wagner is called a "private military company" in Russia and now has international notoriety for alleged war crimes and other abuses in Ukraine.
It has recruited thousands of convicts from prisons after taking heavy losses.
The BBC was unable to verify his claim about serving with Wagner, whose troops have been engaged in intense fighting for months in Bakhmut.
It is rare for a member of the Russian elite to choose to join the group - many have gone abroad to avoid conscription into the regular army.
Nikolai Peskov is also known as Nikolai Choles, and speaks fluent English, having spent several years as a youth in London. He has worked as a correspondent for Russian state broadcaster RT.
Both he and his father are under US sanctions.
In an interview with the pro-Kremlin daily Komsomolskaya Pravda, he said it was his own decision to join Wagner, but he did not know how to do it, "so I had to turn to my dad... and he helped me with that".
He said he used a false ID so that his Wagner comrades would not learn of his Kremlin connections. He did not reveal that assumed name in the interview because, he said, he might need to use it again.
Nikolai Peskov's claim coincides with a major new army recruitment drive, with Russian state ads urging men to do their "patriotic duty" in the Ukraine conflict.
Tens of thousands of men fled Russia last September to avoid being conscripted, after President Vladimir Putin announced a "partial mobilisation".
