Isolated diastolic hypertension (IDH) is frequently considered a lower-risk phenotype due to preserved systolic pressure; however, sustained ambulatory diastolic load may promote early myocardial remodeling. Dynamic circadian variation, particularly diastolic morning surge (DMS), may impose additional hemodynamic stress not reflected by mean blood pressure values. We evaluated whether incorporation of DMS improves risk stratification for subclinical left ventricular diastolic dysfunction (LVDD) in ambulatory-defined IDH. In this cross-sectional study, 737 untreated adults undergoing standardized 24-h ambulatory blood pressure monitoring and echocardiography were analyzed (353 IDH; 384 normotensive controls). IDH was defined as 24-h mean diastolic blood pressure ≥80 mmHg with systolic pressure <130 mmHg, and LVDD was adjudicated using 2016 ASE/EACVI criteria. In adjusted analyses, IDH was independently associated with LVDD (OR 2.79, 95% CI 1.79-3.77). Addition of DMS attenuated but did not eliminate the IDH association and provided independent incremental information (OR 1.44 per 5 mmHg, 95% CI 1.09-1.90). Incorporation of DMS improved discrimination (AUC 0.76 to 0.81; p = 0.041), strengthened calibration, reduced overall prediction error, and provided greater net clinical benefit across intermediate risk thresholds. Ambulatory-defined IDH is strongly associated with subclinical LVDD, and assessment of dynamic diastolic load through DMS offers incremental information beyond mean ambulatory pressure values for model-based discrimination and phenotypic characterization of LVDD.