Background Delirium is an acute neuropsychiatric syndrome characterised by fluctuating course, acute onset, and inattention. Despite being associated with adverse outcomes, including higher mortality, delirium is severely under-diagnosed with as many as 75% of cases missed in acute hospitals. The reasons for under-detection of delirium include lack of reliable measures and symptom overlap with dementia. Recently, the Edinburgh Delirium Test Box (EDTB), employing tasks of sustained attention, has been validated as an objective screening tool for detecting delirium and distinguishing it from dementia. The primary aim of the current study evaluated the feasibility and validity of a smartphone adaptation of the EDTB known as the DelApp. Methods The first phase of this study recruited 18 cognitively normal patients and 2 patients with dementia to evaluate the feasibility of using a smartphone application in the elderly patients. Patients performance on the DelApp was compared to their performance on the EDTB. In the second phase, the DelApp was administered to 3 groups of hospital patients: (1) cognitively normal controls (n=54); (2) patients with dementia but no current delirium (n=52); and (3) patients with delirium (n=50). Delirium was diagnosed using traditional methods. Arousal, depression, cognitive impairment and attention were further assessed using conventional tests. Results First phase of the study indicated that the DelApp is a feasible way of measuring inattention. In the second phase, delirium patients (median=6, IQR=4-7) performed significantly worse on the DelApp than those with dementia (median=10, IQR=9-10; p<.001) and controls (median=10, IQR=10-10; p<.001), which was associated with delirium severity (rs=-0.73, p<0.0001). Receiver operating characteristic analysis indicated that the DelApp was 98% sensitive and 94% specific to delirium. Deficits in sustained attention were also associated with depression and abnormal levels of arousal. Conclusions These results indicate the utility of objective assessment of sustained attention in identifying delirium and differentiating it from dementia. The DelApp showed a promising potential as an objective tool for diagnosing delirium. This research also highlights the importance of controlling for arousal and depression, when assessing inattention in delirium.