Sydnones are mesoionic heterocyclic chemical compounds possessing a C5-oxygenated 1,2,3-oxadiazole core, named after the city of Sydney, Australia. Like other mesoionic compounds they are dipolar, possessing both positive and negative charges which are delocalized across the ring.

N-phenylsydnone was first prepared in 1935 by John Campbell Earl [de] and Alan W. Mackney [Wikidata] by cyclodehydration of N-Nitroso-N-phenylglycine with acetic anhydride. Later work showed that this could be applied fairly generally to the nitrosamines of N-substituted amino acids.

The parent compound sydnone is not synthetically accessible and may not exist.

Sydnones have the following resonance structures.^{[citation needed]} The exocyclic oxygen atom (O6) has a significant negative charge.

Recent computational studies have indicated that sydnones and other similar mesoionic compounds are nonaromatic, "though well-stabilized in two separate regions by electron and charge delocalization."

A sydnone imine in which the keto group of sydnone (=O) has been replaced with an imino (=NH) group can be found as a substructure in the stimulant drugs feprosidnine and mesocarb.