Fulminant type 1 diabetes mellitus (FT1DM) is recognised as a novel subtype of type 1 diabetes mellitus characterised by the abrupt onset of insulin-deficient hyperglycaemia and ketoacidosis. Fulminant type 1 diabetes mellitus is known to be associated with pregnancy and had been associated with high fetal mortality. We report a case of a gestational diabetes mellitus (GDM) mother complicated with FT1DM immediately post-delivery. A 29-year-old Malay lady who was diagnosed with GDM at 19 weeks of pregnancy, underwent emergency lower segment caesarean section (EMLSCS) due to fetal distress at 36 weeks of gestation; 18 h post-EMLSCS, she developed abrupt onset Diabetic ketoacidosis (DKA) (blood glucose 33.5 mmol/L, pH 6.99, bicarbonate 3.6 mmol/L, ketone 4.4 mmol/L and HbA1c 6.1%). She received standard DKA treatment and discharged well. Her plasma C-peptide level 3 weeks later showed that she has no insulin reserve (C-peptide <33 pmol/L, fasting blood glucose (FBS) 28 mmol/L). Her pancreatic autoantibodies were negative. This case highlights that FT1DM not only can occur in pregnancy with normal glucose tolerance but can also complicate mother with GDM.