The `30 Rock` star - who already has a five-year-old daughter Alice with her husband Jeff Richmond - made the announcement as she taped an upcoming episode of `The Oprah Winfrey Show`, with her representative later confirming she is five months pregnant.
Tina - whose real name is Elizabeth Stamatina Fey - met composer Jeff at Chicago`s Second City comedy theatre and the couple dated for seven years before marrying in 2001 with Alice born in 2005.
The 40-year-old actress - whose husband is the composer for `30 Rock` - has previously spoken of the difficulties of juggling raising a childwith the demands of her career.
She said: "I never really believed it before, but when you have a child, there really aren`t enough hours in the day to do anything.
"Basically, any free time that isn`t spent looking after our daughter or working goes on sleeping.
However, Tina admitted in her new memoir `Bossypants` - which was released earlier this week - she felt guilty that Alice doesn`t have a sibling yet.
She wrote: "I thought that raising an only child would be the norm in Manhattan, but my daughter is the only child in her class without a sibling. Most kids have at least two."
"Who will be my daughter`s family when my husband and I are dead from stress-induced cankers?" she cracked. "She must have a sibling."