Denis Rooney, chairman of the Nomadic Charitable Trust which is restoring the vessel, said: "This is a very special day as Nomadic is celebrating her centenary birthday."
The sister ship of the Titanic has celebrated the centenary of its launch.
The SS Nomadic carried passengers to the doomed transatlantic liner, which sank on its maiden voyage in 1912 with the loss of more than 1,500 lives.
At a ceremony in Belfast a commemorative plaque was unveiled on board the Nomadic to mark the date.
Denis Rooney, chairman of the Nomadic Charitable Trust which is restoring the vessel, said: "This is a very special day as Nomadic is celebrating her centenary birthday.
"The Nomadic has played a very important role in the Titanic history and it is nice to honour her as she is the last remaining White Star Line vessel to have survived in the world today."
The ship was launched from Hamilton Dock on April 25, 1911. It was luxuriously equipped and was the tender ship for the Olympic class transatlantic liners.
In January 2006, the Department for Social Development purchased the SS Nomadic at a Paris auction.
The vessel was brought back to it's home city and it is being restored to its former glory in Hamilton Dock, Belfast.
The Special EU Programmes Body, the Northern Ireland Tourist Board and Ulster Garden Villages have also provided funding to help with the restoration.