Simple Summary: Attention-deficit/hyperactivity disorder (ADHD) is a prevalent neurodevelopmental disorder in humans, characterised by inattention, hyperactivity, and impulsivity. Evidence suggests that family dogs can exhibit behaviours similar to symptoms of ADHD seen in humans. While past research mainly used questionnaires to assess ADHD-like behaviours in dogs, we aimed to create a simple behavioural test battery covering key ADHD symptom dimensions: inattention, hyperactivity, and impulsivity. Using our previously validated questionnaire (Dog ADHD and Functionality Rating Scale–DAFRS; 2024) in a study involving 59 dogs, we correlated owner-rated DAFRS scores with behavioural test outcomes to further validate our questionnaire. In a subsample of 38 dogs, we also determined whether there is a difference in how owner- and trainer-rated ADHD scores are linked with the test variables. Our test battery included attention, plush dog, leash, and sit tests, all showing significant correlations with the relevant questionnaire scores in an expected direction. Both owner- and trainer-rated ADHD questionnaire factor scores showed correspondence with the relevant behavioural test variables. Dogs' training status only showed association with the sit test. The test battery proved to be consistent in time; test–retest analyses demonstrated moderate-to-excellent agreement across all tests. Our findings endorse the validity of our questionnaire and behavioural tests, providing valuable tools for assessing inattention, hyperactivity, and impulsivity in dogs. Family dogs exhibit neuropsychological deficits similar to attention-deficit/hyperactivity disorder (ADHD) symptoms in humans. Questionnaire methods have mostly been used to assess ADHD-like behaviours in dogs. In addition to our validated questionnaire (Dog ADHD and Functionality Rating Scale–DAFRS; 2024), we developed a simple behavioural test battery covering the ADHD symptom domains (i.e., inattention, hyperactivity, and impulsivity) in dogs. Our main aim was (i) to provide a final external validation step to the DAFRS by examining its associations with the test variables (N = 59); and (ii) to compare owner- and trainer-rated factor scores' associations with the test variables (n = 38). We developed four tests covering the ADHD symptom domains: the attention test (inattention), the plush dog test (impulsivity), the leash test, and the sit test (hyperactivity). All four behavioural variables correlated with their respective questionnaire scores, i.e., the strongest for hyperactivity, and the least strong for inattention. Both owner- and trainer-rated scores (n = 38) correlated with the relevant test variables in an expected direction. Dogs' training status was linked only to the sit test results. Test–retest analyses (n = 34) indicated moderate-to-excellent agreement across all behavioural variables. Our findings support the validity of our novel human-analogue questionnaire for dogs as the behavioural tests strongly correlate with the relevant questionnaire scores, indicating that the two constructs together can effectively assess inattention, hyperactivity, and impulsivity in dogs. [ABSTRACT FROM AUTHOR]