Relaxor ferroelectrics are applied in electronic devices such as actuators and sonars. Morphotrophic phase boundaries (MPBs) with monoclinic structures are known for their high piezoelectricity and electromechanical coupling factors in solid solutions of PbTiO3 and relaxor ferroelectrics [Pb(Mg1/3Nb2/3)O3 or Pb(Zn1/3Nb2/3)O3]. Using transmission electron microscopy and synchrotron x-ray scattering, we present observations of coexisting monoclinic structures and polar nanoregions near the MPB in Pb(Yb1/2Nb1/2)O3−PbTiO3. The polar nanoregions in this material are randomly shaped, unlike the ferroelectric nanodomains of the canonical relaxor Pb(Mg1/3Nb2/3)O3−PbTiO3. Furthermore, in situ observations reveal that the monoclinic polar nanoregions grow as the temperature decreases. A pair-distribution function analysis reveals a mixture of monoclinic Pm and Cm structures in the polar nanoregions without the rhombohedral structure of other Pb-based relaxor solid solutions. Owing to the peculiar nature of the coexistence of the relaxor property (polar nanoregions) and high piezoelectricity (monoclinic structure), this material is expected as a platform for understanding relaxor ferroelectricity.