Jan 18 (Reuters) - Japan's Mitsui & Co (8031.T) said on Tuesday it had agreed to buy a 14.9 percent stake in Russia's largest instant payment system Qiwi, increasing the conglomerate's presence in the Russian market.

The company did not disclose financial details of the deal, under which Qiwi's management and internet investor Mail.RU Group will cut their stakes to 63.7 percent and 21.4 percent respectively. (Reporting by Anastasia Teterevleva; Writing by Maria Kiselyova; Editing by David Holmes)