Epitaxial strain plays an important role in determining physical properties of perovskite ferroelectric oxide thin films because of the inherent coupling between the strain and the polarization. However, it is very challenging to directly measure properties such as polarization in ultrathin strained films, using traditional sandwich capacitor devices, because of high leakage current. Hence, a planar electrode device with different crystallographical orientations between electrodes, which is able to measure the polarization response with different electric field orientation, is used successfully in this work to directly measure the in-plane polarization–electric-field (P-E) hysteresis loops in fully strained thin films. We used BiFeO3 (BFO) as a model system and measured in-plane P-E loops not only in the rhombohedral-like (R-like) BFO thin films but also in largely strained BFO films exhibiting the pure tetragonal-like (T-like) phase. The exact magnitude and direction of the spontaneous polarization vector of the T-like phase is deduced thanks to the collection of in-plane polarization components along different orientations. It is also shown that the polarization vector in the R-like phase of BiFeO3 is constrained to lie within the (11̅ 0) plane and rotates from the [111] towards the [001] pseudocubic direction when the compressive strain is increased from zero. At high misfit strains such as −4.4%, the pure T-like phase is obtained and its polarization vector is constrained to lie in the (010) plane with a significantly large in-plane component, ∼44 μC/cm2. First-principles calculations are carried out in parallel, and provide a good agreement with the experimental results.