The orthoalkylation catalyst for phenols of the invention is one produced by calcining a catalyst precursor comprising basic magnesium carbonate (a) and magnesium oxide (b) optionally together with manganese oxalate (c), the basic magnesium carbonate (a) and magnesium oxide (b) being mixed together at a weight ratio ((a)/(b)) of 20/80 to 80/20. The process for producing an orthoalkylated phenol according to the invention comprises performing a vapor phase reaction of a phenol with an alkyl alcohol in the presence of the above orthoalkylation catalyst so that an orthoalkylated phenol is obtained.The invention enables obtaining an orthoalkylation catalyst for phenols which has high activity and high selectivity, has long catalytic life and exhibits more stable catalytic life than those of conventional orthoalkylation catalysts for phenols. Moreover, by virtue of the use of the catalyst capable of exerting these effects, there can be obtained a process for producing an orthoalkylated phenol which ensures prolonged and constant catalyst regeneration cycle.