The couple were spotted hand in hand walking along the beach in Malibu, the first time they've been public about their relationship, which is rumoured to have been going on for five years.
The reason for the secrecy is reportedly to do with Katie's ex-husband Tom Cruise, who included a clause in her 2012 divorce settlement banning the Dawson's Creek star from publicly dating for five years, The Daily Mail reports.
Katie and Tom announced their divorce in 2012 after six years of marriage. The couple share an 11-year-old daughter, Suri.
Fans have reacted to the much overdue declaration of love with hilarious Twitter posts.