Cleric Salman, who heads the largest opposition formation in the Sunni-ruled kingdom, had his jail sentence increased on Monday to nine years from the original four.
He had been convicted of inciting violence but the appeals court overturned an earlier acquittal of advocating regime change by force.
The appeals court "increased the sentence despite strong evidence his initial trial was unfair and the fact that two of the charges on which he had been convicted violated his right to freedom of expression", HRW said in a statement.
"This level of repression will not create stability for Bahrain, but quite the opposite," he warned.
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Salman was originally convicted in July 2015, drawing condemnation from rights groups as well as both the United States and Iran.
The tiny but strategic Gulf state has been shaken by unrest since it crushed a month-long, Shiite-led uprising demanding reforms in 2011.
The Shiite-majority kingdom, connected to Saudi Arabia by a causeway, lies across the Gulf from Shiite Iran and is home to the US Fifth Fleet.
Salman's Al-Wefaq political association was Bahrain's largest parliamentary bloc until its 18 MPs walked out in February 2011 in protest at the use of violence against demonstrators.