Harold Wolferstan Thomas (1875–1931) was a Canadian medical doctor, noted for his research in the field of tropical medicine.
Thomas worked in Germany (including at Göttingen and Munich) and at the Montreal General Hospital.
From 1903, he worked at the Liverpool School of Tropical Medicine (LSTM) and from 1904 was head of a laboratory in Runcorn.
Along with Anton Breinl (1880-1944), he discovered in 1905 that the arsenic preparation Atoxyl killed trypanosomes, the causative agent of sleeping sickness.
The studies were performed by infecting laboratory animals such as mice, dogs and monkeys with Trypanosoma brucei gambiense and then using of Atoxyl to effect a cure or delay the progression of the disease.