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Exoplanets can evolve significantly between birth and maturity, as theiratmospheres, orbits, and structures are shaped by their environment. Youngplanets ($<$1 Gyr) offer an opportunity to probe the critical early stages ofthis evolution, where planets evolve the fastest. However, most of the knownyoung planets orbit prohibitively faint stars. We present the discovery of twoplanets transiting HD 63433 (TOI 1726, TIC 130181866), a young Sun-like($M_*=0.99\pm0.03$) star. Through kinematics, lithium abundance, and rotation,we confirm that HD 63433 is a member of the Ursa Major moving group($\tau=414\pm23$ Myr). Based on the TESS light curve and updated stellarparameters, we estimate the planet radii are $2.15\pm0.10R_\oplus$ and$2.67\pm0.12R_\oplus$, the orbital periods are 7.11 and 20.55 days, and theorbital eccentricities are lower than about 0.2. Using HARPS-N velocities, wemeasure the Rossiter-McLaughlin signal of the inner planet, demonstrating thatthe orbit is prograde. Since the host star is bright (V=6.9), both planets areamenable to transmission spectroscopy, radial velocity measurements of theirmasses, and more precise determination of the stellar obliquity. This system istherefore poised to play an important role in our understanding of planetarysystem evolution in the first billion years after formation.

TESS Hunt for Young and Maturing Exoplanets (THYME) III: a two-planet system in the 400 Myr Ursa Major Group