Paul Williams, 43, keeps bees as a hobby and had one hive in a ditch in a field in Rhydwyn on Anglesey.
He checks on it weekly, but found the Â£400 hive and the 30,000-40,000 bees inside, had been taken sometime between 26 July and 2 August.
Mr Williams, of Caergeiliog, said: "I was shocked - the whole hive had gone, you don't expect that."
PC Dewi Evans of North Wales Police rural crime team, which is investigating the theft, said: "The fact there were thousands of bees inside suggests they were wearing a full bee suit or they risked being stung to smithereens."
No arrests have been made.
Mr Williams keeps bees with Gary Jones and Peter Bull, and they have about 15 hives between them.
PC Evans said the thief must have had some knowledge of bees or beekeeping "otherwise they wouldn't have done it".
Also, the hive was not visible from any road, so Mr Williams said he doubted it was done by an opportunistic passer-by.
Each hive has a queen bee which the others bees are loyal to and protect, so Mr Williams said if the thief had disturbed the hive, the swarm would have attacked.
"You'd have to know what you're doing. Some people wouldn't go near them - the first thing they think of is the sting," he said.
"It's not the sort of thing you have in your back garden."
This is not the first time sticky-fingered thieves have pinched a beehive on Anglesey - in 2015, one farm suffered two thefts in the space of a month.
A thief - most likely dressed in a full beekeeping suit - has stolen a beehive containing up to 40,000 of the insects.