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Well‐defined ruthenium(II) phosphinous acid (PA) complexes enabled chemo‐, site‐, and diastereoselective C−H functionalization of arenes and alkenes with ample scope. The outstanding catalytic activity was reflected by catalyst loadings as low as 0.75 mol %, and the most step‐economical access reported to date to angiotensin II receptor antagonist blockbuster drugs. Mechanistic studies indicated a kinetically relevant C−X cleavage by a single‐electron transfer (SET)‐type elementary process, and provided evidence for a PA‐assisted C−H ruthenation step.