Robert Sinclair († 1398) was a late 14th century bishop of Orkney and bishop of Dunkeld.
Before becoming a bishop, he was Dean of Moray and had obtained a Bachelor's degree in Law. By 28 November 1383 he is being spoken of in the documents of Avignon Pope Clement VII as bishop-elect of Orkney, and was probably fully appointed by 27 January 1384. On 1 February 1391 he was translated to the more prestigious bishopric of Dunkeld. Within a few years of his translation to Dunkeld, Robert found himself involved in a serious dispute with William Blackburn, the abbot of Cambuskenneth. The details of this dispute are not entirely known, but on 25 March 1393 he was excommunicated by a papal judge-delegate. Intervention by King Robert III probably gained him absolution within the following few year or so. On 18 January 1395 he visited Cambuskenneth Abbey with a retinue of 56 horses, and was entertained there, an event which certainly points to some kind of reconciliation. Robert was dead by November 1398.