Lee had been jailed for 10 months in the past for a similar offence, The Korea Herald reported.
In its ruling on Monday, a Seoul court said he "continued to generate and disseminate a considerable amount of propaganda that glorified and praised the North", the Korea Herald said.
He posted comments online praising North Korea's military in 2013, while posting anti-state content on South Korean blogs and websites in subsequent years.
South Korea's National Security Act outlaws the praise and promotion of "anti-government" organisations.