The 27-year-old singer confirmed the news on Twitter with a picture of himself and girlfriend Aimee Jade Azari, reported Daily Mirror.
Azari also tweeted the image.
Williams will be the third member of JLS to become a father. Marvin Humes and his wife Rochelle welcomed daughter Alaia-Mai last year, while JB Gill recently announced his wife Chloe Tangney is also expecting a baby.
Humes congratulated the couple on Twitter by writing, "Huge congrats to Oritse and AJ Azari the Williams baby is on the way! Very excited for you both! Big love."
Humes is now working as a TV and radio presenter, recently hosting the third series of 'The Voice UK' with Emma Willis.
Meanwhile, Gill and Merrygold are both working on solo material, while Williams has established a management company and record label and recently fronted Britain's Youngest Carers for Channel 4.