The Sunni-ruled kingdom's court of cassation reduced Salman's sentence from nine to four years, the source said on condition of anonymity.
No further details were immediately available.
The court of cassation in October had ordered a retrial of the 51-year-old cleric, who headed the now-banned Al-Wefaq Shiite opposition movement.
Salman had been arrested on charges of inciting hatred and insulting the state in December 2014.
He was found guilty in July 2015 and sentenced to four years in jail.
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Salman is considered a moderate who has pushed for a constitutional monarchy in Bahrain compared to more hardline groups who have demanded the toppling of the Sunni Al-Khalifa dynasty in a string of protests that go back to 2011.
Bahrain has cracked down over the past six years on dissent by members of the Shiite majority, whom it accuses of being manipulated by Iran.
Shiite Iran has consistently denied any involvement.
The kingdom last year ordered the dissolution of Al-Wefaq, the country's main Shiite opposition party, for "harbouring terrorism".
Its lawmakers resigned en masse in protest against the state crackdown on the 2011 protests.
In 1995, Salman was among a string of oppositionists exiled from Bahrain, moving to the United Arab Emirates and then Britain.
In 2001, he returned to Bahrain under a general amnesty and set up Al-Wefaq National Islamic Society with other Shiite opposition figures.
In 2006 he was elected secretary general of the group.