The Association for Democratic Reforms said 35% of the legislators – 252 of the 690 – had cases pending.
The ruling Congress party fared poorly in state elections that have just been held in Uttar Pradesh, Punjab, Uttarakhand, Manipur and Goa.
These polls are seen as a litmus test for national elections due by 2014.
North-eastern Manipur state is the only exception: not a single elected candidate there is facing criminal charges, the report says.
In addition, the number of candidates with criminal records in the other four states saw an 8% rise over the last state elections in 2007, the watchdog says.
The independent study was based on sworn statements filed by the candidates before they contested the polls.
Uttar Pradesh, India’s largest state, provided the largest number of candidates with criminal charges against them, the analysis shows.
A total of 189 legislators – or 47% of the total – had criminal cases against them in the state, according to the affidavits filed by them, the report says.
In 2007, 140 – or 34% – of the legislators had criminal records.