Background: In young patients, up to 40% of ischemic strokes remain cryptogenic despite modern-day diagnostic work-up. There are limited data on blood pressure (BP) behavior in these patients. Thus, we aimed to compare ambulatory blood pressure (ABP) profiles between young patients with a recent cryptogenic ischemic stroke (CIS) and stroke-free controls. Patients and Methods: In this substudy of the international multicenter case–control study SECRETO (NCT01934725), 24-hour ambulatory blood pressure monitoring (ABPM) was performed in consecutive 18–49-year-old CIS patients and stroke-free controls. The inclusion criteria were met by 132 patients (median age, 41.9 years; 56.1% males) and 106 controls (41.9 years; 56.6% males). We assessed not only 24-hour, daytime, and nighttime ABP but also hypertension phenotypes and nocturnal dipping status. Results: 24-hour and daytime ABP were higher among controls. After adjusting for relevant confounders, a non-dipping pattern of diastolic blood pressure (DBP) was associated with CIS in the entire sample (odds ratio, 3.85; 95% confidence interval, 1.20–12.42), in participants without antihypertensives (4.86; 1.07–22.02), and in participants without a patent foramen ovale (PFO) (7.37; 1.47–36.81). After excluding patients in the first tertile of the delay between the stroke and ABPM, a non-dipping pattern of DBP was not associated with CIS, but a non-dipping pattern of both systolic BP and DBP was (4.85; 1.37–17.10). In participants with a PFO and in those without hypertension by any definition, no associations between non-dipping patterns of BP and CIS emerged. Conclusions: Non-dipping patterns of BP were associated with CIS in the absence of a PFO but not in the absence of hypertension. This may reflect differing pathophysiology underlying CIS in patients with versus without a PFO. Due to limitations of the study, results regarding absolute ABP levels should be interpreted with caution.Key Messages Nocturnal non-dipping patterns of blood pressure were associated with cryptogenic ischemic stroke except in participants with a patent foramen ovale and in those without hypertension by any definition, which may indicate differing pathophysiology underlying cryptogenic ischemic stroke in patients with and without a patent foramen ovale. It might be reasonable to include ambulatory blood pressure monitoring in the diagnostic work-up for young patients with ischemic stroke to detect not only the absolute ambulatory blood pressure levels but also their blood pressure behavior.