CCTV will be examined to try and identify who tipped something into the water of the Market Square fountain in the town centre over the weekend.
The fountain in Market Square, which recently reopened after a £50,000 refurbishment, was pictured filled with bubbles this morning (Monday).
Northampton Borough Council said the same thing happened on Saturday.
A Northampton Borough Council spokesman said: “We were alerted earlier this morning to the bubbles in the fountain, which have been caused by someone adding something to the water deliberately. We can confirm that this also happened on Saturday.
We have already been out this morning to clean up and will be examining CCTV to try to identify the person who has done this and report them to the police. Quite apart from the costs involved from cleaning up after this sort of anti-social behaviour, we know that people enjoy the fountain and it is a shame that their enjoyment is spoiled in this way.”
The fountain was designed to attract more shoppers to Northampton market.
The original fountain, which cost around £98,000 four years ago, was turned off in 2012 after traces of legionella were found in the water.
Northampton Borough Council consulted with the public and the majority of those who replied said they wanted the fountain to be improved and switched back on.