Bangladesh, Feb. 9 -- A Hong Kong court on February 9 sentenced pro-democracy media tycoon Jimmy Lai to 20 years in prison under the citys sweeping national security law, delivering one of the harshest penalties yet imposed on a prominent critic of Beijing and marking a decisive moment in the erosion of press freedom in the semi-autonomous Chinese territory.

Lai, the founder and former publisher of the now-closed Apple Daily newspaper, was convicted in December of conspiring to publish seditious materials and conspiring to “collude with a foreign country or external elements.” The charges stem from editorial decisions and political activities that prosecutors argued were designed to provoke international pressure against Hong...