A series of heavy lanthanide(III) and yttrium(III) monoporphyrinato complexes formulated in [Ln(TPP)(cyclen)]Cl (Ln = Tb, Dy, Ho, Er, Tm, Yb, and Y; TPP = 5,10,15,20-tetraphenylporphyrinato), with cyclen, 1,4,7,10-tetraazacyclododecane, as a capping ligand, have been prepared in mild conditions and studied using single-crystal X-ray diffraction crystallography. The complexes exhibit an electronic absorption band (B(0,0)) in the range of 421-423 nm, showing a bathochromic shift associated with the increase of the ionic radii of the lanthanide, as well as two peaks of Q(1,0) and Q(0,0) bands between 548-586 nm. All of the complexes are isostructural, where both TPP and cyclen are coordinated to a lanthanide(III) or yttrium(III) ion giving an eight-coordinate square-antiprismatic (SAP) geometry (average skew angles are in the range of 43.01°-43.67°). The mean plane of the four nitrogen atoms of TPP (N4t) and that of the cyclen (N4c) are virtually parallel with a dihedral angle of less than 1°. The lanthanide(III) or yttrium(III) ions lie between N4t and N4c. The position of the metal ion is closer to the N4t plane, which is presumably caused by the different charges of the ligands, the size of the N4 square ligands, and the steric factor. The average Ln-N and interplanar distances (dN) decrease with decreasing lanthanide(III) ionic radii, showing the effect of lanthanide contraction. The skew angles, opening angles, and N-N distances are nearly unchanged, keeping the rigid square antiprismatic geometry throughout the series.