Robert Johnston, or Johnson (died 1687) was an English-born politician and judge in seventeenth-century Ireland.
Henry Bennet, 1st Earl of Arlington acknowledged him as his cousin and furthered his career.
After the Restoration of Charles II, he accompanied Sir Edward Smith, a judge of the new Court of Claims, to Ireland as his secretary.
Smith canvased the electors on his behalf as being "a worthy gentleman", and he also had the support of his cousin Lord Arlington, a rising man in the English Government.
He was married: his eldest son Robert Johnson junior was, like his father, an MP and a High Court judge.