Jolovan Wham was convicted last month of breaking public order laws after he failed to apply for a police permit before inviting activist Joshua Wong to speak via video call to an audience.
The 39-year-old was also found guilty of refusing to sign a written statement to police. On Thursday he was handed a fine of Sg$3,200 ($2,300).
Wham, who is currently out on bail, said he will appeal the sentence, but if that fails he said he will choose to serve a 16-day jail term rather than pay the fine.
The activist, who is also an advocate for migrant workers' rights, had described the gathering in November 2016 as "a harmless and straightforward discussion about social movements".
Wong, who was among the leaders of massive pro-democracy protests in Hong Kong in 2014, spoke to the audience about the role of civil disobedience and democracy in building social movements.
Organising a public assembly without a permit in Singapore is punishable by a fine of up to Sg$5,000. Repeat offenders can be fined up to Sg$10,000 or jailed for a maximum of six months or both.
Wealthy Singapore is regularly criticised by rights groups for its heavy-handed response to political dissent and freedom of expression.
In December, the editor of a Singaporean website was charged with defamation for publishing a letter alleging corruption among the country's leaders.
The same month, Prime Minister Lee Hsien Loong sued a blogger for defamation after he shared an article on Facebook linking the leader to a corruption scandal in neighbouring Malaysia. Lee said the article was false and without basis.