The chief director of the Bahrain Center for Human Rights was due to hear his fate on Thursday but the 51-year-old will now remain detained until 28 December.
He has been in pre-trial detention, much of it in solitary confinement, since his arrest in June as part of a state crackdown on dissidents.
Rajab's court appearance lasted 15 minutes in which the judge refused to allow Rajab to speak, Index on Censorship reported.
"Bahrain's repeated postponement of Nabeel Rajab's trial is emblematic of its wider approach to the human rights of its citizens," said Melody Patry from the campaign group.
"Arbitrary trials, extra-judicial detentions and outright stripping of citizenship are the Bahraini government's favourite weapons to silence dissenting voices. Nabeel's continuing detention is unjust, cruel and a blatant violation to his right to freedom of expression.
"We call on Bahraini authorities to release him immediately and unconditionally."
Rajab is being prosecuted on multiple charges of "disseminating false rumours in time of war", "insulting a neighbouring country" and "insulting a statutory body" over tweets he posted about the humanitarian crisis caused by the Saudi-led war in Yemen - with Saudi Arabia the "insulted" country - and documenting torture in Bahrain's Jau prison.
He was first expected to be sentenced in October 2016, but the court instead reopened his case for investigation.
Rajab's extended detention has caused a deterioration in his health, according to his family.
|This is a farcical trial which represents the human rights crisis facing the Gulf|
Pressure has also mounted on the UK government this week to follow the US, EU and UN in urging Bahrain to release Rajab.
Ahead of Thursday's court appearance, 23 British MPs signed a joint letter calling on the UK government to demand Rajab's "unconditional release" from prison.
The letter, signed by a cross-party group of MPs, calls on Prime Minister Theresa May to speak out over Bahrain's human rights record.
Margaret Ferrier, a Scottish MP who chairs Westminster's All-Party Parliamentary Group on Democracy and Human Rights in the Gulf, said: "This is a matter of freedom of speech and expression. Nabeel Rajab is highly-regarded as a prominent human rights activist, and these charges are little more than an attempt by Bahraini authorities to silence a dissenting voice.
"The Foreign Office has given assurances that they have repeatedly raised Mr Rajab's case with Bahraini officials, and whilst this is welcome, we are calling for it to go one step further and urge its key Gulf ally to release him unconditionally and to drop all charges."
"This is a farcical trial which represents the human rights crisis facing the Gulf," said Sayed Ahmed Alwadaei, from the Bahrain Institute of Rights and Democracy.
"Bahrain's allies, in particular the US and UK, must publicly call for his release. In particular, the UK has still not called for his release, nor used its leverage to help this clear human rights violation. Failing to do so will help perpetuate injustice, which undermines any potential long-term stability."