A member of the public discovered them at Festival Park in the Pacific Quay area of Glasgow on February 20 and the Scottish SPCA were alerted.
However, the person who found the tennis balls placed them in a bin and they were then removed by someone before animal welfare officers got there.
The charity said it is possible they were left to target rats, but warned of a danger to dogs and children.
An undercover inspector with the charity’s special investigations unit said: “Someone had cut holes in the tennis balls and filled them with shredded paper and blue pellets, thought to be rat poison.
“The person who found them placed them in a nearby bin but unfortunately someone took them before we could remove them. Police Scotland have been made aware of this.
“It’s possible the balls were left to target rats, but there is an obvious danger to dogs and children and this was a very reckless act.
“We are looking for information as to why the balls were in the park and who may be responsible. We are also keen to identify the person who took the tennis balls from the bin.”
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