We have measured Raman scattering and high‐resolution x‐ray diffraction from highly strained [(InAs)4(InP)4]Nshort‐period superlattices grown on InP substrates by atomic layer epitaxy at 355 °C. The InAs and InP confined phonons are observed in these highly strained short‐period superlattices. The energy of the InAs confined longitudinal‐optical phonon (LO) modes of a fully strained superlattice (withN=8) is blue shifted by about 10 cm−1compared to the LO phonon of bulk InAs. This effect is explained by the large biaxial strain existing in the InAs layers. The observed frequency shift agrees with the lattice‐mismatch strain given by elasticity theory and independently measured by high‐resolution x‐ray diffraction. No evidence of a frequency shift of the InP confined LO modes in theN=8 fully strained superlattice is observed, indicating that the strain is confined to the InAs layers. We show that in a partially relaxed superlattice (withN=20), the InAs layers are in compression, while the InP layers are in tension. In this case the InP confined LO1phonons are red shifted by about 3 cm−1compared to the InP LO1phonons of theN=8 fully strained superlattice, and the shift of the InAs confined LO phonons with respect to the LO phonons of bulk InAs is also reduced.