The LHCb Collaboration has reported resonant activity in the channel D+K-, identifying two components: X0(2900) with JP=0+ at 2866±7 MeV, Γ0=57±13 MeV and X1(2900) with JP=1- at 2904±7 MeV, Γ1=110±12 MeV. We interpret the X0(2900) component as a csūd¯ isosinglet compact tetraquark, calculating its mass to be 2863±12 MeV. This is the first exotic hadron with open heavy flavor. The analogous bsūd¯ tetraquark is predicted at 6213±12 MeV. We discuss possible interpretations of the heavier and wider X1(2900) state and examine potential implications for other systems with two heavy quarks.