The previous record was set in 2023 at 15C, the agency said in a social media post late on Monday.
Spain is considered a frontline region for climate change, experiencing increasingly long heatwaves that sometimes start before summer, along with more frequent episodes of intense rainfall.
Aemet described April as "extremely warm", noting it was 3.2C above the 1991–2020 average. Six days in April set new daily heat records, it added.
Spain experienced multiple wildfires in April, mainly in the north, linked to dry weather and unusually warm temperatures.
The warm April followed Spain's wettest January and February in 47 years.
Eleven major storms swept the country from late December to mid-February, bringing heavy rain and strong winds.
Spain sweltered through its hottest summer on record last year, with an average temperature of 24.2C.
Europe and the rest of the world could face another extremely hot summer as the El Niño weather phenomenon, which pushed global temperatures to record highs in 2024, is expected to return in the middle of the year.
Scientists have repeatedly warned that human-driven climate change is resulting in more frequent and intense extreme weather events worldwide.
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