China's crude imports from Iran rose 16.1% in June over a year earlier to 780,175 barrels per day, up from 671,176 barrels in May. Imports for the January-June period also gained 2.5%.
Shipments have held relatively steady as Tehran has been focusing on recouping lost markets in Europe after sanctions were lifted, Iranian oil sources say, Reuters reported.
Meanwhile Saudi Arabia, the world's biggest oil exporter, regained its position as China's top crude supplier in June, after losing out to Russia over the previous three months, customs data showed.
China imported 4.569 million tons of crude from Saudi Arabia in June, or 1.112 million bpd, down 14.2% on the year but beating 961,000 bpd in May.
Saudi imports edged up 0.24% in the first six months of the year versus a year ago to an average of 1.06 million bpd.
Russian exports to China have benefited from good demand by independent refiners since late 2015 after the country allowed them to import crude for the first time.
China imported 4.107 million tons, or around 999,420 bpd, of crude in June from Russia, down from a record 1.24 million bpd in May. Russian imports rose 35.3% in the first half to 1.05 million bpd, just behind Saudi Arabia.
"Beijing is probably quite pleased with the competition for shares of China's crude oil market," said Washington-based China energy expert Erica Downs of the Eurasia Group.
"The government doesn't want to be too dependent on any one supplier, so competition between major suppliers is a welcome development, especially if it results in lower prices."
Nicknamed "teapots" due to their relative smaller scale, independents contributed more than half of China's 930,500 bpd incremental crude purchases during the first half.
Stockpiling to boost government reserves was another driver for imports, as new tanks became available.
