Spanish road law (Reglamento General de Circulación, art. 118) requires an approved helmet for every cyclist on interurban roads — which is where you spend almost all of your riding time in Mallorca, so plan on wearing one every ride. Within towns (poblado) the helmet is only mandatory for riders under 16 and for people who ride professionally.
Until 30 September 2026 three narrow exemptions let a rider over 16 go bare-headed on an interurban road: prolonged ascending ramps, conditions of extreme heat, and accredited medical reasons. For Mallorca that mattered, because the first two described a summer Tramuntana ascent exactly.
Since 1 October 2026 those exemptions are gone. Real Decreto 518/2026, de 24 de junio (BOE of 26 June 2026) makes the helmet compulsory for every cyclist on an interurban road, and, for anyone who works on a bike, adds a fastened-helmet duty on urban roads and a high-visibility reflective vest on every type of road. The rule for under-16s on every type of road stays as it was.
The duty sits in the road-traffic regulation, the fine in the Ley de Tráfico (RDL 6/2015): not using a mandatory helmet is a grave infraction (art. 76.h), and a grave infraction carries a €200 fine (art. 80.1).