Henderson has put pen to paper on what the Merseyside club have described as a long-term deal, having agreed personal terms and visited the Reds Melwood training complex on Wednesday before undergoing a medical.
A statement on Liverpools official website read: "Liverpool Football Club have completed the signing of Jordan Henderson after the 20-year-old passed a medical and agreed personal terms.
"Henderson travelled to Merseyside on Wednesday after a fee was agreed with Sunderland. He then underwent a medical examination in Liverpool before signing a long-term deal.
"The midfielder has now joined the England Under-21 squad in Denmark as they prepare for the European Championships.''
Henderson's first game after completing his transfer is likely to be against Spain Under-21s on Sunday.
It had been thought Reds striker David Ngog might travel in the opposite direction as part of the transfer, but for now the Frenchman remains at Anfield.