Nicholas Barnewall, 3rd Viscount Barnewall (15 April 1668 – 14 June 1725) was an Irish nobleman.
His grandfather, also named Nicholas Barnewall, had been ennobled by King Charles I on 12 September 1645 for loyalty to his cause. Nicholas was the eldest son of Henry, 2nd Viscount Barnewall, by Lady Mary Nugent, daughter of Richard Nugent, 2nd Earl of Westmeath. Before Nicholas was of age he married Mary Hamilton, daughter of George, Count Hamilton (son of Sir George Hamilton, 1st Baronet, of Donalong), by his wife, Frances Jennings (afterwards married to the Earl of Tyrconnel). They had three children: Henry, 4th Viscount Barnewall; George, father of the 5th Viscount; and Frances, who married her distant cousin Richard Barnewall and was the mother of the 14th Baron Trimlestown.
In 1688 he entered King James's Irish army as captain in the Earl of Limerick's Dragoons. After the defeat of the Boyne he was moved to Limerick; and being in that city at the time of its surrender, was included in the articles and secured his estates. In the first Irish Parliament of William III he took the oath of allegiance, but upon declining to subscribe the declaration according to the English Act of 1689, as contrary to his conscience, he was obliged to withdraw with the other Catholic lords. In February 1703, he joined with many Irish Catholics in an unavailing petition against the infraction of the Treaty of Limerick. Lord Barnewall died 14 June 1725, and was buried at Lusk.