In what is the sixth incident of its kind in just over a year, the Scottish SPCA described the act as 'abominable'.
The first report of the contaminated biscuits in Glasgow was made last year when they were discovered in the Anderston area of the city.
It was followed by two further reports from the public made in March.
On three separate occasions since July 8, people have reported seeing them in Dorset Square.
Scottish SPCA Inspector Vickery said: "We are grateful that no animal was harmed by the biscuits and we are thankful to the vigilant members of the community who alerted us to these incidents.
"In total, this is the sixth incident of its kind we have had reported to us in the Anderston area in just over a year.
"This is a calculated and abominable criminal offence and we will do all we can to find the person responsible. It seems they won't stop until an animal is hurt.
"Someone knows something and we are desperate to put a stop to this. If anyone has any information on these incidences or sees anything suspicious, please report it immediately."
In April, a leading criminologist said the series of attempts to poison dogs in Glasgow parks could be the work of a psychopath.
Professor David Wilson, the founding director of the Centre for Applied Criminology at Birmingham City University, said people who target pets may have a disorder called Zoosadism, meaning they take pleasure in making animals suffer.