The reactivity of low‐valent alkyl‐substituted metallacyclobutadienes towards electron‐rich alkynes was studied. Treatment of the alkyl‐substituted rhenacyclobutadiene complexes Re{−C(R)=C(CO2Et)C(OEt)=}(CO)4 (R=Me, Et) with HC≡COEt produced a mixture of the cyclopentadienyl complexes Re{η5‐C5H(R)(CO2Et)(OEt)2}(CO)3 and the new rhenacyclobutadienes Re{−C(OEt)=C[C(R)=CH(CO2Et)]C(OEt)=}(CO)4 bearing a pendant vinyl substituent C(R)=CH(CO2Et) at the β‐carbon. The reactions proceed through initial nucleophilic attack of the terminal carbon of HC≡COEt on the ReC(R) carbon of the starting rhenacyclobutadienes. The rhenacyclobutadiene complex Re{−C(Me)=C(CO2Et)C(OEt)=}(CO)4 reacted with aminoalkynes RC≡CNEt2 (R=Ph, Me) to produce the unusual coupling products Re{−C(OEt)=C(CO2Et)C[CH=C(NEt2)CH2R]=}(CO)4, metallacyclobutadiene complexes bearing the vinyl substituent CH=C(NEt2)CH2R at an α‐carbon. The mechanisms of the reactions are discussed.