The refurbishment is part of a wider improvements project, which will see further announcements before the end of the year.
The project coincides with the 15th anniversary of Hilton Cardiff, which first opened its doors in August 1999.
The refurbishment sees the hotel's bedrooms redecorated and furnished in a muted palette of whites and creams with bright cobalt feature elements and metallic fittings.
Marie Fagan, general manager of Hilton Cardiff, said: "This is an incredibly exciting time for Hilton Cardiff and we are very pleased to welcome the first guests to our new and improved rooms."
Hilton Cardiff has views over the medieval castle and the Edwardian civic centre and offers seven meeting spaces, catering for between 20-350 theatre-style.