Signe "Ham" Hammarsten-Jansson (née. Hammarsten, 1 June 1882 Hannäs, Sweden - 6 June 1970 Porvoo) was a Swedish graphic artist who designed, among other things, around 220 Finnish postage stamps during the course of three decades. She was the mother of Tove Jansson, Per Olov Jansson and Lars Jansson.
Signe was the main basis for Moominmamma. Her death greatly influenced Tove in the writing of Moominvalley in November, which was the last Moomin novel. She is the direct inspiration for Sophia's grandmother in The Summer Book. Moominland Midwinter was dedicated to her.