Sydney Mary Thompson (also known as Madame Christen; 1847 – 16 July 1923) was an Irish geologist, botanist and artist.

She described her childhood as "amphibious"; her family home was at Macedon Point and she frequently explored the coastline with her brother in a small boat.

[3] An active member of the Belfast Naturalists' Club, Thompson organised lectures and practicals, showing a particular interest in stratigraphy and petrography.

[6] Some time between 1907 and 1910, Thompson discovered a piece of Ailsa Craig microgranite at Moys, near Limavady, County Londonderry.

Thompson's discovery placed the western limit of the glacier 20 miles (32 km) further west than its previous designation.