Sales of just 1,669 made the track the 154th biggest selling single last week but, crucially, brought up the October 2010 release up past the seven-figure mark, according to Official Chart Company data.
The 108th UK single to record a million unit sales, Only Girl (In The World) is the 15th single by a female solo artist to pass the milestone - the oldest being Julie Covington's 1977 chart-topper Don't Cry For Me Argentina, and the most recent Adele's January 2011 single Someone Like You.
Interestingly, the 107th million seller - which reached the target just six weeks ago - is Eminem's Love The Way You Lie, on which Rihanna is credited as the featured vocalist.
Barbadian Rihanna is only the second female solo artist not from the UK or North America to earn the honour, the first being Denmark's Whigfield, with 1994 debut hit Saturday Night.
Canada's Celine Dion is the only female solo artist to have two songs on the list, selling a million copies of Think Twice and another million of the Titanic movie theme, My Heart Will Go On.
Trailing Only Girl (In The World) as Rihanna's biggest seller as primary artist are What's My Name (feat. Drake). which has sold 768,669 copies to date and Umbrella (feat. Jay-Z), which has sold 763,068 - but both tracks are likely to be overtaken this week by Rihanna's current smash We Found Love, which features Calvin Harris, and has sold 732,896 copies in nine weeks.
Selling upwards of 50,000 copies every week, We Found Love topped the chart for six weeks and seems certain to become Rihanna's second solo million seller at some point in 2012.
The only other artists to have more than two million sellers are The Beatles.
Rihanna has sold 12,022,665 singles in the UK since her August 2005 debut Pon De Replay - more than any other artist in the same timeframe.