Runners at north Wales’ largest running event will be disqualified if they drop litter during the race.
Organisers of Sunday’s Conwy Half Marathon said the measures had been introduced as plastic was becoming an “increasing problem”.
The Run Wales website said runners will also be “taken off the results if seen discarding their rubbish outside of a water stop or not with a marshal”.
The race is in its 11th year and will host 3,000 runners.
The Conwy half marathon started in front of the town’s castle at 10:00 GMT, with the route taking in nearby Llandudno.
Several roads have been closed for the event.
Run Wales is following a growing trend among race organisers to reduce the environmental impact of events.
In April 2019, the London Marathon trialed several measures to reduce litter after clearing 47,000 plastic bottles from the streets in 2018.
These included using compostable cups rather than plastic bottles at some stations along the route, and using plastic bottles made wholly or partly from recycled plastic.
This year’s Cardiff Half Marathon used 100% recyclable plastic bottles, recycled paper for all print advertising and medals made from recycled zinc.