Nearly half of all car seats checked during a child safety initiative were found to be incorrectly fitted, incompatible with the vehicle or unsuitable for the child.
More than half of parents said they had not been shown how to put the seats in properly before setting off on journeys, the research found.
Director of Road Safety Scotland, Michael McDonnell, said retailers should be providing installation advice to parents buying new car seats.
"It is very disappointing to find that many retailers are still not providing basic safety information when selling child car seats," he said. "I would strongly urge parents and carers to visit an in-car child safety clinic so they can be confident their children are safely restrained."
The findings, part of the Scottish Good Egg In-Car Child Safety Initiative, also showed that more than 10% of parents are not confident about their ability to fit seats safely.
The figures come after road casualty statistics published last week by Transport Scotland found five children were killed last year while travelling as car passengers.
Running for 10 years, the Good Egg campaign aims to ensure babies and children are properly restrained while travelling in the car.
Its findings, which involved assessing nearly 2000 car seats in Scotland, were supported by the Association of Chief Police Officers in Scotland.
Superintendent Alan Duncan said: "We are still finding children who are either not wearing seatbelts or who are restrained by unsuitable or incorrectly-fitted car seats."
The charity is today launching 54 free in-car child safety clinics around Scotland, which will run for the next three months, offering checks on car seats.
Sir Arnold Clark, chairman and chief executive of the Arnold Clark Group, has sponsored the Good Egg campaign each year since its national launch in 2002.
He said: "It doesn't take much time to check that a car seat is fitted properly and it will always be too late after a collision.
"I therefore urge parents to go to a Good Egg clinic to ensure that their child seats are fitted safely – it could ultimately save their child's life."
In the UK, the legal requirement is that children under 12 – or less than 135cm (4'5'') tall – must use a car or booster seat while children under three travelling in a vehicle must be carried in a correct child restraint, either a baby seat or child seat.
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