Italian theologian whose anti-Trinitarian views were developed into the doctrine of Socinianism (Socinian) by his nephew Faustus Socinus.

Born of a distinguished family of jurists, Laelius was trained in law at Padua but turned to biblical research, which ultimately led him to doubt the Roman Catholic church's dogmas. At 21 he went to Venice, and afterward he traveled in Switzerland, France, England, and Holland. In 1548 he settled in Zürich to study Greek and Hebrew. The following year he corresponded with John Calvin on doctrinal matters, and the next year he was the guest of the German religious reformer Philipp Melanchthon at Wittenberg. Wherever he went Socinus conducted his theological inquiries, with special concern for the sacraments, grace, predestination, resurrection of the body, repentance, and the doctrinal basis of the original gospel. Rumours began to spread that Socinus was a heretic, and, after a warning from the Swiss Reformer Heinrich Bullinger, he composed a confession of faith (July 1555) that seemed orthodox but that also left open the door to heretical views. He spent his last years at Zürich.

A highly speculative thinker, Socinus reached few well-defined conclusions, but his theological views survived in the fully developed system of his nephew Faustus, whom he strongly influenced.