Men who have a history of smoking cannabis have higher sperm counts and fertility than those who don't.
The result came as a big surprise to scientists measuring the fertility of more than 600 men from couples attending a clinic.
They expected cannabis to have a detrimental effect on sperm count and fertility.
Instead, those participants who admitted ever taking the drug turned out to have higher sperm counts than non-users.
The finding does not necessarily mean that smoking cannabis increases the chances of fatherhood, the study authors and other experts were quick to point out.
There could be a non-causal explanation for the association, such as the effect of the male hormone testosterone on both sperm count and risk-taking behaviour such as smoking cannabis.
US lead researcher Dr Jorge Chavarro, from the Harvard TH Chan School of Public Health in Boston, said: "These unexpected findings highlight how little we know about the reproductive health effects of marijuana, and in fact of the health effects of marijuana in general.
"Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use."
Previous research involving animal experiments or men with histories of drug abuse has suggested that cannabis impairs male reproductive health.
For the new study, investigators collected 1,143 semen samples from 662 men between 2000 and 2017.
On average the men were 36 years old, mostly white and college educated. All of them belonged to couples seeking help with conception from a fertility clinic.
Participants were asked to fill in questionnaires detailing their history of cannabis use.
More than half (55%) of the men reported having smoked cannabis at some point. Of those, 44% said they had taken the drug in the past, and 11% classified themselves as current users.
Analysis of the semen samples showed that men who had smoked marijuana had average sperm concentrations of 62.7 million sperm per millilitre (million/mL).
Those who had never smoked a joint had an average count of 45.4 million/mL.
Only 5% of cannabis users had sperm counts below 15 million/mL, the World Health Organisation's threshold for "normal" levels, compared with 12% of men who had never smoked cannabis.
In addition, greater cannabis use was linked to higher levels of the male hormone testosterone among smokers.
Writing in the journal Human Reproduction, the scientists said it was possible that low level exposure to cannabis might benefit sperm production in some way.