The 22-year-old, who is out of contract at Posh in the summer, has made seven appearances for the Spireites since joining the League Two title chasers on an initial month-long deal in March.
"I've enjoyed it since I've been here. The staff and the boys have been brilliant with me and made me feel really welcome," Kearns told Chesterfield's official website.
"I'm happy to get the loan extended to the end of the season, the main aim is to get promotion and then hopefully push on to try and win the title."
Chesterfield have the best goal difference in League Two, but they are currently third in the overall table, two points behind leaders Scunthorpe United.