Isotactic and syndiotactic poly(methyl methacrylate)s with orthogonally double-clickable terminal ends, that is, α,β-unsaturated esters for Michael addition-type thiol–ene reactions and azide or alkynyl groups for azide–alkyne click reactions, were prepared via a terminating reaction of stereospecific anionic polymerization with propargyl and 2-chloroethyl α-(chloromethyl)acrylates. The subsequent polymer modification via a double click reaction proceeded quantitatively in a one-pot system under ambient conditions. The facile and almost quantitative double-end-functionalization would open a new material design based on stereoregular PMMAs with controlled molecular weights.