Denise Van Outen originally landed the role but dropped out from the programme this afternoon because of "other contractual commitments".
Walsh, who previously hosted Live From Studio Five, has now joined the celebrity news show.
In a statement, Channel 5 said: "We are delighted to announce that Kate Walsh will be stepping up to present OK! TV with Matt Johnson.
"We all know how fantastic and experienced Kate is following her time on Live From Studio Five and are delighted to have her on board to co-present this fantastic new show."
OK! TV begins on Monday at 6.25pm on Channel 5.