The "low three-figure" sum of money was taken from three purses which had been left unattended at the Aberdeen Sea Cadets base at Pocra Quay.
Insp Carron McKellar said: "This is a disgraceful incident which has left the victims shocked and upset."
Anyone with information on the theft is urged to contact Police Scotland.
Insp McKellar added: "It is understood that the suspect entered the building while visitors were in another part of the building and took the money. Nothing else has been taken."
The theft of money from purses at a Remembrance Sunday event at Aberdeen harbour has been branded "disgraceful" by police.