Countess Izabella Poniatowska (1 July 1730 – 14 February 1808) was a Polish noblewoman, sister of king Stanisław August Poniatowski.

She was described as her brother's close confidante and friend, and her relationship with him is described as sentimental and tender, but she was said to lack any understanding of - or interest - in political issues.

She lived a life in comfort and had a private Italian orchestra, theater and ballet, and spent the winters in Warsaw and the summers in Bialystok.

She and her sister Ludwika Maria Poniatowska both opposed her brother's suggested marriage to princess Sophia Albertina of Sweden.

She stayed with her brother the King in Warsaw all through the Kościuszko Uprising to support him, despite being afraid, and the first shots during the revolution is said to have been fired near her palace.