In another break-in thieves stole a pile of jewellery before helping themselves to a couple of pints of milk and some cans of Coke from the fridge.
Detective Inspector Nigel Oliver, from the priority crime unit based in Southsea, said both incidents were unexplained.
'Feeding a cat during a burglary is not common at all,' he said.
'It may have been to keep the cat in a particular part of the house.'
The burglary happened between 2pm and 9pm on Sunday, March 22, in South Parade, Southsea.
After feeding the homeowner's pet the raider stole a box of fireworks and left.
Det Insp Oliver said: 'The man was absolutely certain that the sachet of cat food had been opened for the cat and we're not exactly sure what the motive for that was, it's very unusual.
'You see films where a guard dog is slipped a bit of meat but this is unusual.
'Normally a burglar's intention is to get in and out as quickly as possible with property they can grab.'
In a separate incident, intruders smashed a conservatory door to get into a house in St Paul's Road, Southsea, between 11.50am on Friday, March 20 and 12.15am on Saturday, March 21.
Once inside they stole a silver Lenovo laptop worth £250; a silver DVD player worth £20; CDs and DVDs; two Folli pearl necklaces; a Tiffany and Co bracelet; tear-shaped diamond earrings; and a necklace and earring set.
But while laptops and jewellery are common prey for thieves, they also stole a pair of ladies' high-heeled shoes and items from the fridge including two pints of full-fat Co-op milk, three cans of Coke and a bottle of Jack Daniels.
Det Insp Oliver said: 'Again this is fairly unusual for the same reason.
'People have this idea of burglaries where extensive searches are done and everything is tipped upside down but more often they are opportunistic where the burglar grabs something like a laptop and then they are out again.
'To rifle through the fridge is a little bit more common than feeding the cat but it's unusual.'
Anyone with information on either incident should call police in Portsmouth on 08450 45 45 45.