Nearly half of the top performing schools in Wales are 'socially selective' and accept fewer disadvantaged pupils.
The 40 state schools where pupils are most likely to get five good GCSEs, admit almost half the the number of disadvantaged pupils than the average state school.
While some of the gap is due to catchment area policy, the trust says a large proportion of it is due to schools being selective about pupils.
It says the percentage of disadvantaged pupils given a space in the top schools does not reflect the local community.
The Sutton Trust says Welsh medium cater to 'more advantaged' families.
All Welsh medium schools have a lower proportion of disadvantaged pupils, whether they are the the top performing or not.
The Charity is calling for changes in the admissions process, including prioritising disadvantaged pupils and providing better information and outreach to parents in deprived areas.
It says it will help improve access to the best school in Wales.
The report also recommends that local authorities, particularly in urban areas, should consider a system with fewer incentives for middle class parents to buy homes in the catchment areas of top schools.