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The two-point function for tensor metric perturbations around de Sitterspacetime including one-loop corrections from massless conformally coupledscalar fields is calculated exactly. We work in the Poincar\'e patch (withspatially flat sections) and employ dimensional regularization for therenormalization process. Unlike previous studies we obtain the result forarbitrary time separations rather than just equal times. Moreover, in contrastto existing results for tensor perturbations, ours is manifestly invariant withrespect to the subgroup of de Sitter isometries corresponding to a simultaneoustime translation and rescaling of the spatial coordinates. Having selected theright initial state for the interacting theory via an appropriate i\epsilonprescription is crucial for that. Finally, we show that although the two-pointfunction is a well-defined spacetime distribution, the equal-time limit of itsspatial Fourier transform is divergent. Therefore, contrary to the well-defineddistribution for arbitrary time separations, the power spectrum is strictlyspeaking ill-defined when loop corrections are included.

One-loop gravitational wave spectrum in de Sitter spacetime