Sivarajah Jeneevan was sentenced last January to 10 years in jail for conspiring to murder Sirisena when he was irrigation minister in 2005.
The president welcomed Jeneevan onto the stage where he was making a speech to mark the anniversary, shaking hands with the former rebel and blessing him by touching him on his head.
Sri Lanka thanks Pakistan for help in wiping out terror
Sirisena came to power promising "compassionate rule and stable country", and has already released a number of hard core rebels from the Liberation Tigers of Tamil Eelam (LTTE), who fought for secession for more than 25 years until their defeat in 2009 under Sirisena's predecessor, Mahinda Rajapaksa.
Sirisena's opponents have, however, accused him of weakening national security with the releases.
Pakistan seeks closer military ties with Sri Lanka
Sirisena has also promised to end corruption, rebalance foreign policy by reducing Rajapaksa's focus on China, and ensure the independence of the judiciary and civil service.
In line with UN recommendations, his government has pledged to establish a credible judicial process involving foreign judges to investigate allegations of war crimes during the bloody climax of the war against the LTTE.
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