The world is experiencing unprecedented water related issues. Due to climate change, over-abstraction and pollution, water related problems will continue to increase, contributing to water insecurity. In consequence, water governance is undergoing a paradigmatic transformation from reductionist, top down, engineering approaches to more integrated ones featuring inclusive institutions and adaptive management predicated at the river basin scale. Here, globally leading water governance mechanisms such as the EU Water Framework Directive (WFD) and the California Sustainable Groundwater Management Act (SGMA) aim to counter water insecurity through integrated river basin management. However, comparative evaluations of the effectiveness of these different institutional arrangements for achieving water security are limited. The aim of this thesis is therefore to comparatively assess these two different forms/models of integrated river basin management for achieving water security outcomes, to inform policy learning. Meeting this aim involves meeting five objectives, namely: (i) identifying relevant gaps in the water security literature through critical review; (ii) developing a methodological approach for assessing how integrated river basin management supports water security; (iii) theoretically comparing different forms of integrated water governance, using a modification of Ostrom’s IAD framework, to examine key institutions, processes and outcomes; (iv) comparatively assessing the extent to which these two governance models support water security through the use of a dedicated assessment tool; and (v) making policy recommendations on lesson-drawing for future integrated river basin planning, to better support water security globally. To meet these objectives, quantitative and qualitative research methods are utilized to examine the effectiveness of integrated river basin planning within an embedded comparative case study design. This thesis concludes that WFD implementation in the Konya Closed Basin in Turkey only partially leads to water security while asserting that the Kern County Subbasin in California shows stronger institutional capacity for supporting water security in the implementation of SGMA. Recommendations for enhancing water security through integrated river basin management therefore include mechanisms for increasing institutional capacity through collaboration between agencies, more inclusive public participation, and better data collection and characterization of groundwater data, monitoring of plan implementation and document preparation.