Jean Denis, comte Lanjuinais (12 March 1753 - 13 January 1827) was a French politician.

He was born at Rennes (Ille-et-Vilaine). After a brilliant college career, which made him doctor of laws and a qualified barrister at nineteen, he was appointed counsel to the Breton estates and in 1775 professor of ecclesiastical law at Rennes. At this period he wrote two important works which, owing to the distracted state of public affairs, remained unpublished, Institutiones juris ecciesiastici and Praelectiones juris ecclesiastici.

He had begun his career at the bar by pleading against the feudal droit du colombier , and when he was sent by his fellow-citizens to the states-general of 1789 he demanded the abolition of nobility and the substitution of the title of king of the French and the Navarrese for king of France and Navarre, and helped to establish the civil constitution of the clergy. Returned to the Convention in September 1792 he developed moderate, even reactionary views, becoming one of the fiercest opponents of the Mountain, though he never wavered in his support of republican principles. He refused to vote for the death of Louis XVI, alleging that the nation had no right to despatch a vanquished prisoner.

His daily attacks on the Mountain resulted, on the 15th of April 1793, in a demand by the commune for his exclusion from the assembly, but, undaunted, when the Parisian populace invaded the Chamber on the 2nd of June, Lanjuinais renewed his defiance of the victorious party. Placed under arrest with the Girondins, he escaped to Rennes where he drew up a pamphlet denouncing the constitution of 1793 under the curious title Le Dernier Crime de Lanjuinais (Rennes, 1793). Pursued by J. B. Carrier , who was sent to stamp out resistance in the west, he lay hidden until some time after the revolution of Thermidor (July 1794), but he was readmitted to the Convention on the 8th of March 1795.

He maintained his liberal and independent attitude in the Conseil des Anciens , the Senate and the Chamber of Peers , being president of the upper house during the Hundred Days. Together with G. J. B. Target , J. E. M. Portalis and others he founded under the empire an academy of legislation in Paris, himself lecturing on Roman law. Closely associated with oriental scholars, and a keen student of oriental religions, he entered the Academy of Inscriptions in 1808. After the Bourbon restoration, Lanjuinais consistently defended the principles of constitutional monarchy, but most of his time was given to religious and political subjects. Besides many contributions to periodical literature he wrote, among other works, Constitutions de la nation francaise (1819); Appreciation du projet de loi relatif aux trois concordats (1806, 6th ed. 1827), in defence of Gallicanism; and Etudes biographiques et littraires sur Antoine Arnauld, P. Nicole et Jacques Necker (1823). He died in Paris.

His son, Victor Ambroise, vicomte de Lanjuinais (1802-1869), was also a politician, becoming a deputy in 1838. His interests lay chiefly in financial questions and in 1849 he became minister of commerce and agriculture in the cabinet of Odilon Barrot. He wrote a Notice historique sur la vie et les ouvrages du comte de Lanjuinais, which was prefixed to an edition of his fathers OEuvres (4 vols., 1832).

For the life of the comte de Lanjuinais see also A. Robert and G. Cougny , Dictionnaire des parlementaires, vol. ii. (1890); and F. A. Aulard , Les Orateurs de la Legislative et de la Convention (Paris, 1885 1886). For a bibliography of his works see J. M. Qurard , La France littraire, vol. iii. (1829).