We study stabilization of finite-dimensional representations of the periplectic Lie superalgebras p(n) as n → ∞.The paper gives a construction of the tensor category Rep(P ), possessing nice universal properties among tensor categories over the category sVect of finite-dimensional complex vector superspaces.First, it is the "abelian envelope" of the Deligne category corresponding to the periplectic Lie superalgebra, in the sense of [EHS15].Secondly, given a tensor category C over sVect, exact tensor functors Rep(P ) → C classify pairs (X, ω) in C where ω : X ⊗ X → Π½ is a non-degenerate symmetric form and X not annihilated by any Schur functor.The category Rep(P ) is constructed in two ways. The first construction is through an explicit limit of the tensor categories Rep(p(n)) (n ≥ 1) under Duflo-Serganova functors. The second construction (inspired by P. Etingof) describes Rep(P ) as the category of representations of a periplectic Lie supergroup in the Deligne category sVect ⊠ Rep(GL t ).An upcoming paper will give results on the abelian and tensor structure of Rep(P ).Contents. Introduction 1 2. Preliminaries and notation 4 3. The infinite periplectic Lie superalgebra 9 4. The subcategories Rep k (p(n)) 12 5. The DS functor 17 6. Construction of the category Rep(P ) 22 7. The first universal property 26 8. The second universal property 29 9. Lower highest weight structure 36 References 39 10. Appendix A: Direct summands of tensor powers 40 11. Appendix B: Affine group schemes in tensor categories 43