The warplanes arrived disassembled aboard a massive Antonov An-124 transport plane that landed at a military airport in the Belgrade suburb of Batajnica on October 2, according to RFE/RL.
The MiGs are being provided by Moscow at no charge, but their assembly, repair, and refurbishing costs are expected to near $235 million in total.
The four other jets are set to arrive sometime before October 20.
Read alsoSurveillance photos 'show Russian intelligence officers plotting Montenegro coup' – The TelegraphIn March, Serbia's Aleksandar Vucic, then prime minister and now president, confirmed earlier reports that Russia would provide the six MiG-29 fighters "as a gift."
He added that the planes would enter military service by the end of 2017 after refurbishing and pilot training.
Russia also committed to provide 30 T-72 tanks and 30 BRDM-2 patrol combat vehicles to Serbia for free.
Read alsoU.S. envoy urges Balkan States to guard against Russian influence efforts – mediaVucic said on October 2 that Serbia also planned to improve its defenses and has been negotiating with Moscow for the purchase of Russian-made S-300 antiaircraft systems.
Vucic, a former nationalist, has remade himself as a pro-European Union reformer while seeking to maintain good relations with traditional ally Russia, which is looking to block the Balkan nation's path toward possible NATO membership.
Serbia's moves to heighten military ties with Moscow have worried the West and many neighboring countries, including Bosnia-Herzegovina and NATO member Croatia.
A NATO official told the Associated Press that "the defense equipment which NATO's partners procure is a sovereign choice for those countries. There are no restrictions imposed by NATO."