According to the oscillation theory of handwriting, letters are formed by the coupled modulation of horizontal and vertical oscillation of our hands. This modulation-oscillation scheme requires human motor control. Hence one of the reasons for poor handwriting is lack of fine motor skills. The main objective of this study is to design, develop and evaluate a hand therapeutic-application (HanDex) for children with writing difficulties, to enhance their hand dexterity. Hence this paper describes the design, development and evaluation of the HanDex app. It has mainly five activities for improving tripod grip, hand-eye coordination, spatial organization, letter formation and fine motor skills. The main principle behind the design of all these activities is the oscillation theory of handwriting. A set of user interface (UI) guidelines have been followed in the UI design for simplicity, user-motivation, easy navigation, etc. User evaluation of the prototype was conducted by ten participants in five iterations and the final product was built by considering their feedback. After that, we have conducted case studies in two primary school children with poor handwriting to test the effectiveness of the application. The study used a single subject pretest-posttest design to observe and measure the efficacy of the HanDex application. The post test results showed a positive impact on letter formation, size of letters, spacing of letters and words, placement, speed and legibility in these children. Hence the HanDex app is promising and warrants further investigation on more children with different levels of handwriting difficulty.INDEX TERMS Digital tools, evolutionary prototyping, fine-motor skill, HCI principles, handwriting difficulty, hand dexterity, Tablet PC, touch-based interfaces.