Lebanese security said the pro-Assad man was leaving his work in bustling Hamra street in the early hours on Wednesday when the dissident approached him and stabbed him with a spear.
"It seems they knew each other and the dissident was watching him. He arrived on a motorbike and then when he saw him he stabbed him," a security source said.
Thousands of Syrians fleeing the violence raging in their country have sought refuge in Lebanon.
Lebanon's government has distanced itself from the conflict, fearing it might spill into the country that is divided along sectarian lines and where Syria has powerful allies as well as enemies.
The northern city of Tripoli has been the scene of fierce fighting between Lebanese factions taking up opposing sides of the Syrian conflict.
For decades, Syria had far-reaching influence on its smallest neighbour. Assad withdrew Syrian troops from Lebanon in 2005 but the powerful Shi'ite Muslim militant group Hezbollah remains one of its strongest allies in the country.