Andy Hall was convicted after raising concerns about alleged labour rights abuses in the lucrative Thai seafood sector.
The MEPs are among 60 civil society organisations, 28 unions and worker organisations and 13 companies expressing concern over Thailand's use of criminal defamation to prosecute human rights defenders.
The protest letter was timed to coincide with International Human Rights Day.
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Hall was given a suspended jail sentence for defaming a pineapple wholesaler in a report alleging labour abuses at a Thai seafood products firm.
The UK activist, who had campaigned for the rights of migrant workers in Thailand's fruit industry, was found guilty of defamation and computer crimes.
Hall had contributed to a report by a Finnish watchdog, Finnwatch, in 2013 alleging the Natural Fruit Company mistreated its workers.
The alliance, including the cross party group of MEPs, say the Hall case "gives cause for real concern."
The letter to the Thai regime says, "We are writing to you as an international coalition of civil society groups (human rights, labour, development and environmental organisations), national civil society groups, members of Parliament, and corporations who seek to ensure that the rights of migrant workers and human rights defenders in Thailand are respected and protected in line with international law and standards."
It goes on, "While we acknowledge the decision of the Bangkok South Criminal Court in this case, we remain deeply troubled about the potential of this judgment to seriously hinder the work of human rights advocates by preventing effective and confidential research and monitoring of supply chains, thereby putting migrant and other vulnerable workers at higher risk of debt bondage, forced labour and other abuse."
The alliance state, "Without basic rights like free association and collective bargaining, migrant workers in Thailand lack the means to effectively protect themselves from abuse and exploitation. This judgment could put them at even greater risk."
They urge Thailand to repeal the provisions in the penal code criminalising defamation and amend the "computer crime act" to bring it into compliance with international human rights law regarding freedom of expression.
The MEPs are Heidi Hautala, Sirpa Pietikäinen, Lisa Jaakonsaari, Merja Kyllönen, Miapetra Kumpula-Natri, Nils Torvalds, Pirkko Ruohonen-Lerner and Glenis Willmott.