The 26-year-old Sharapova has qualified for the year-end finale for the seventh time in her career but her participation remains in doubt due to her continuing recovery from injury, which forced her to sit out the US Open.
Sharapova and Radwanska join the already-qualified duo of Serena Williams and Victoria Azarenka in the line-up, with four more spaces still up for grabs.
Sharapova won the event on her debut in 2004, defeating Serena Williams in the championship match, and is a two-time finalist, in 2007 and most recently 2012. Only once has she failed to reach at least the semi-final stage, withdrawing from round-robin play in 2011 with an ankle injury.
Radwanska, 24, reaches the WTA Championships for the third successive year, as well as competing as an alternate in 2008 and 2009. Her best result came last year, reaching the semi-finals, before falling to eventual champion Serena Williams.
Doubles duo Sara Errani and Roberta Vinci have also reached the tournament after ending the 2012 season as WTA Doubles Team of the Year.
“We are excited to add Maria, Agnieszka, Sara and Roberta to the WTA Championships player field, although Maria continues to rehab from her injury,” said Stacey Allaster, WTA chairman and CEO.
“We are less than a month away from the championships and are eager to continue building towards what will be another sensational tournament for the fans in Istanbul and tennis fans around the world.”
The WTA Championships showcases the world’s best women tennis players as the top eight singles players and top four doubles teams compete for $6m in prize money and one of the most prestigious titles in women’s tennis.
Istanbul is set to host the elite season-ending competition for a third and final year.