The Poverty Alliance was commissioned to examine the public transport system by Transport Scotland
The report recommends expanding free bus travel to all under-25s and those receiving low-income benefits.
In March, the Scottish Government announced all under-22s will receive free bus travel.
Research based on focus groups with low-income families found the cost of public transport is often prohibitive and causing social isolation.
Peter Kelly, director of the Poverty Alliance said: "These findings support what communities have been telling us for many years; that too many families in Scotland are locked into hardship because of our transport system.
"In the just and compassionate society we all want to live in, our public services should help secure a decent life for everyone.
"Yet as this report makes clear, right now our transport system is tightening rather than loosening the grip of poverty."
Transport minister Graeme Dey welcomed the report.
He said: "The findings from this research highlight the urgent need for us to develop actions to address the interlinked challenges of public transport availability and affordability on a specific and targeted basis.
"We are already working to introduce the legal changes required to extend free bus travel to everyone under the age of 22.
"Building on this, I will ensure that further consideration is given to addressing the issues around the availability of public transport services, and I look forward to working with the Poverty Alliance to take forward the findings of this research."