Clijsters, who retired for a second time after the 2012 U.S. Open, announced the news via Twitter, and so did her husband.
"Jack Leon Lynch is born! Born at 7.06 pm tonight and we are both doing well... His big sister , mama and daddy are very proud and happy!❤" she tweeted.
"Jack Lynch is born! Absolutely incredible. Mom @Clijsterskim is well and is my hero! Jada is an adorable big sis. What more can I say!!" Lynch wrote.
Clijsters, 30, welcomed her first child, a daughter named Jada, on February 27, 2008.
Avril Lavigne denies racism in 'Hello Kitty' music video
Humongous iceberg slowly drifts away from Antarctica