Johnathon Richardson had a discussion with his female colleague in 2022 during which she admitted that "creepy crawlies" made her squeamish.
The tribunal heard this planted a seed in his mind, and he left the exoskeleton of a tarantula in her pigeonhole.
He told the tribunal he had been trying to shock her, but that this would be followed by "light-hearted relief" when she realised the skin was not a real spider. He had not expected her to get upset.
Richardson mentioned the prank when he next met her and she called him a "f*cking twat". He then suggested he might do something similar with a snakeskin and she replied that she would report him.
Richardson then left a snakeskin in the same colleague's pigeonhole, after which he was reported for bullying and eventually fired for gross misconduct.
However, the tribunal found he had been unfairly dismissed.
Employment judge Matthew Hunt said: “The sort of prank performed... was plainly very ill-judged but extremely unlikely in reality to have led to serious impacts.
"They didn't involve any risk of physical harm to [his colleague], they were not of an abusive nature, they were largely harmless, childish pranks.”
Richardson's compensation will be decided at a later date.
Aisling Foley, employment solicitor at law firm SAS Daniels, said there is often a fine line between banter and bullying in the workplace.
She told HR magazine: “It is important for both employers and employees to understand that there is a line when it comes to banter and pranks in the workplace, as it can be easy for that kind of behaviour to stray into bullying territory.
“This case is a good reminder of how what one person thinks is a joke or a prank may not land that way with another person.”
She added that employers should lay out clear bullying and harassment policies to ensure staff behave appropriately.
She added: “The workplace should be a safe place for all employees and with more and more people suffering from stress and increased anxiety, it is extremely important for employers to ensure their staff feel supported, but also that they understand what won’t be tolerated.
“Line management training and ensuring there are clear policies in place will help to ensure staff are clear on how they should be behaving at work and should also promote a more positive workplace culture.”