Jupiter is the most massive planet of the Solar System and its presence had an immense effect on the dynamics of the solar accretion disk. Knowing the age of Jupiter, therefore, is key for understanding how the Solar System evolved toward its present-day architecture. However, although models predict that Jupiter formed relatively early, until now, its formation has never been dated. Here we show through isotope analyses of meteorites that Jupiter’s solid core formed within only ∼1 My after the start of Solar System history, making it the oldest planet. Through its rapid formation, Jupiter acted as an effective barrier against inward transport of material across the disk, potentially explaining why our Solar System lacks any super-Earths.