The present disclosure relates to a coaxial connector and a board-to-board connector assembly. The coaxial connector comprises an outer conductor, an inner conductor, and a dielectric spacer disposed between the outer conductor and the inner conductor. The outer conductor includes a first outer conductor component and a second outer conductor component configured to be floatable axially and radially relative to the first outer conductor component. The inner conductor includes a first inner conductor component and a second inner conductor component configured to be floatable axially and radially relative to the first inner conductor component. The coaxial connector further includes a first elastic element disposed around an outer circumference of the proximal portion of the second outer conductor component, and a second elastic element disposed in the cavity of the first inner conductor component. The coaxial connector according to the present disclosure is self-adaptive for the mating, and is particularly suitable for board-to-board connector assemblies and may ensure a high return loss performance and good PIM characteristics.