The buf CLI can do this.
Here's how it looks to convert from encoded protobuf to json (protojson).
Here's how it looks to convert from encoded protobuf to json (protojson).
buf convert schema.proto \
--type YourMessageName \
--from payload.binpb \
--to output.json
buf convert schema.proto \
--type YourMessageName \
--from data.json \
--to encoded.binpb
[ my public key: https://keybase.io/sudorandom; my proof: https://keybase.io/sudorandom/sigs/lhEkaUpV37r7QzVea_cLcElODrYrTIMZezP1qIJjCro ]