Nabeel Rajab was charged with insulting a public institution and the army.
In the tweet, he suggested that security institutions in Bahrain had served as what he called an “ideological incubator” for jihadists.
The court has granted Mr Rajab bail of $500 (£430) while he appeals against the verdict, if he chooses to pay it.
Mr Rajab has served several prison sentences since setting up the Bahrain Centre for Human Rights in 2002.
He confirmed his latest sentencing in a tweet that read: “I was sentenced to 6 month.”
In an interview with the BBC he said that the laws and judiciary in Bahrain were being used as tools of repression.
Blow to freedom
He compared his sentence to “a sword at his neck” to prevent him from speaking, and also accused the British government of keeping silent about the case.
Meanwhile Amnesty International described the sentence as “a blow to freedom of expression”.
Rights groups say Mr Rajab’s conviction is the latest move against activists by authorities in Bahrain.
On Monday, the head of the country’s main opposition group was charged with promoting violent regime change.
Bahrain’s Sunni Muslim rulers forcibly quelled a 2011 uprising which had been triggered in large part by unrest among the Shia Muslim majority.
However sporadic demonstrations have continued, and on Tuesday there were further clashes between activists and police in the capital Manama.