The Okaz newspaper reported on Tuesday that the demonstrators were part of 21-strong group arrested in the Red Sea city of Jedda on 16 December "for taking part in a gathering".
The sentences ranged from one to six-month prison terms coupled with a pledge not to repeat such action, it said.
The 15 will also receive between 100 and 200 lashes "depending on the act committed by each defendant and whether he repented", the daily added.
Political demonstrations are outlawed under the strict laws of Saudi Arabia.
The demonstrations were called for by the London-based Movement for Islamic Reform (MIRA) but were thwarted as Saudi security forces deployed en masse in Riyadh and Jedda.
The group's chief, Saad al-Faqih, had called for peaceful protests in support of ending monarchic government, which it accuses of mass corruption.
Ending monarchic government is
a stated goal of Saudi dissidents
Al-Faqih told Aljazeera.net on on Tuesday that all MIRA's broadcasts into Saudi Arabia were now being jammed and that various international agencies were attempting to prevent him from making any public criticism of Riyadh.
Nevertheless, he said his organisation had succeeded in getting tens of thousands of Saudis to pray at particular mosques on Friday in a successful attempt to promote discussions about the government and the need for reform.
MIRA managed to organise demonstrations last year in several cities of the oil-rich ultra-conservative kingdom, leading to the arrest of more than 300 people, mainly in Riyadh.