Portnoy said he will catch several dates after the Fifty Something tour opens on 7 June at Los Angeles' Kia Forum, with Geddy Lee and Alex Lifeson returning.
Fusion drummer Anika Nilles will sit in for the late Neil Peart, and the North American leg runs through December before South America and Europe dates in early 2027.
Portnoy, who said he became friends with Peart in his final decade, called Rush one of his favourite bands and said fans still need classic acts onstage.
With Rush's tour weeks away, can new drummer Anika Nilles truly honor Neil Peart's legacy while forging her own?
Geddy Lee hints at new music. Will this reunion tour be a farewell tribute or the dawn of a new Rush era?