This paper presents a theory of excitonic polarons that is amenable to first-principles calculations. Excitonic polarons are quasiparticles formed by the interaction of an exciton (a bound electron-hole pair) with the surrounding lattice, leading to localization. The theory is developed using a combination of the Bethe-Salpeter equation (BSE) and linear-response calculations of exciton-phonon couplings. The key advantage of this approach is that it does not require supercells, allowing for the study of a wide range of materials hosting either small or large excitonic polarons. The theory is first applied to model Hamiltonians for Wannier excitons with Fröhlich or Holstein electron-phonon couplings. It is found that in the case of Fröhlich interactions, excitonic polarons only form when there is a significant difference between the effective masses of the electron and hole. The theory is then applied to calculate excitonic polarons in lithium fluoride (LiF), demonstrating its effectiveness in real materials. The formalism is derived by expressing the total energy of a distorted lattice in a neutral excited state as the sum of its total energy in the electronic ground-state and the BSE excitation energy. The total energy is minimized by solving a coupled nonlinear eigenvalue problem, which involves the exciton wavefunction and atomic displacements. The solution is obtained by expanding the wavefunction in a transition basis and transforming it into an exciton basis, which significantly reduces the computational cost. The theory is validated by comparing the results with known models and by demonstrating its applicability to both model systems and real materials. The results show that the formation of excitonic polarons can be understood as a two-step process: first, the formation of independent electron and hole polarons, and second, their binding through mutual Coulomb attraction, accompanied by the weakening of lattice distortion due to partial cancellation of the electron and hole charge densities. The paper concludes that the proposed theory provides a robust framework for calculating excitonic polarons in real materials, and it highlights the importance of considering both the electronic and lattice degrees of freedom in the description of these quasiparticles. The approach is particularly useful for studying materials with strong electron-phonon coupling, where the formation of self-trapped excitons is a key phenomenon.This paper presents a theory of excitonic polarons that is amenable to first-principles calculations. Excitonic polarons are quasiparticles formed by the interaction of an exciton (a bound electron-hole pair) with the surrounding lattice, leading to localization. The theory is developed using a combination of the Bethe-Salpeter equation (BSE) and linear-response calculations of exciton-phonon couplings. The key advantage of this approach is that it does not require supercells, allowing for the study of a wide range of materials hosting either small or large excitonic polarons. The theory is first applied to model Hamiltonians for Wannier excitons with Fröhlich or Holstein electron-phonon couplings. It is found that in the case of Fröhlich interactions, excitonic polarons only form when there is a significant difference between the effective masses of the electron and hole. The theory is then applied to calculate excitonic polarons in lithium fluoride (LiF), demonstrating its effectiveness in real materials. The formalism is derived by expressing the total energy of a distorted lattice in a neutral excited state as the sum of its total energy in the electronic ground-state and the BSE excitation energy. The total energy is minimized by solving a coupled nonlinear eigenvalue problem, which involves the exciton wavefunction and atomic displacements. The solution is obtained by expanding the wavefunction in a transition basis and transforming it into an exciton basis, which significantly reduces the computational cost. The theory is validated by comparing the results with known models and by demonstrating its applicability to both model systems and real materials. The results show that the formation of excitonic polarons can be understood as a two-step process: first, the formation of independent electron and hole polarons, and second, their binding through mutual Coulomb attraction, accompanied by the weakening of lattice distortion due to partial cancellation of the electron and hole charge densities. The paper concludes that the proposed theory provides a robust framework for calculating excitonic polarons in real materials, and it highlights the importance of considering both the electronic and lattice degrees of freedom in the description of these quasiparticles. The approach is particularly useful for studying materials with strong electron-phonon coupling, where the formation of self-trapped excitons is a key phenomenon.