Dame Kathleen Lonsdale DBE FRS (née Yardley; 28 January 1903 – 1 April 1971) was an Irish crystallographer, pacifist, and prison reform activist. She proved, in 1929, that the benzene ring is flat by using X-ray diffraction methods to elucidate the structure of hexamethylbenzene. She was the first to use Fourier spectral methods while solving the structure of hexachlorobenzene in 1931. During her career she attained several firsts for female scientists, including being one of the first two women elected a Fellow of the Royal Society (FRS) in 1945 (along with Marjory Stephenson), first female professor at University College London, first woman president of the International Union of Crystallography, and first woman president of the British Association for the Advancement of Science.

She was born Kathleen Yardley in Newbridge, County Kildare, Ireland. She was born to English-born Harry Yardley, the town postmaster, and Jessie Cameron, a Baptist of Scottish descent.

She was the youngest of ten children, four of whom died in infancy. During her time living in Newbridge she attended St. Patrick's National School, and her earliest memories were of the local Church of Ireland service and the Methodist Sunday school.

Kathleen's father had issues with alcohol, which meant her family was often short on money. As the political unrest in Ireland became more severe, Kathleen's mother separated from her father and took the rest of the family to England in 1908.

Her family moved to Seven Kings, Essex, England, when she was five years old. The family's financial troubles meant the four older children left school early to support the family. For the same reason, her brother Fred was unable to take up an educational scholarship, though he later become one of the first wireless operators.

Kathleen attended Downshall Elementary school from 1908 to 1914. She studied at Ilford County High School for Girls, then transferred to Ilford County High School for Boys to study mathematics and science, because the girls' school did not offer these subjects. Kathleen achieved the highest score in physics that any student at London University ever had. She graduated with a Bachelor of Science (BSc) degree from Bedford College for Women in 1922, and Master of Science (MSc) degree in physics from University College London in 1924.

In 1924 she joined the crystallography research team headed by William Henry Bragg at the Royal Institution. Following her marriage in 1927, she moved to the University of Leeds, but continued to correspond with Bragg.

While at Leeds between 1927 to 1932, she started a family but also set up X-ray equipment using a grant from the Royal Society. She balanced her work on the determination of space groups with the task of looking after her children. While at Leeds the Professor of Chemistry, Christopher Ingold suggested that she investigate the crystal structures of hexamethylbenzene and hexachlorobenzene. In both cases she showed the molecules to have a planar, hexagonal structure settling the long-standing dispute about the structure of benzene. Her husband Thomas Lonsdale was a textile chemist who supported his wife's research. He encouraged his wife to work from home and to go back to work when offered. He worked at the Silk Research Association in Leeds after they were married.