Background: Early numeracy interventions including basic arithmetic are crucial for young students at risk for early mathematics difficulties (MDs), yet few studies have evaluated numeracy interventions in second grade with a randomized controlled design. Aim: This pre- and post-test randomized controlled study evaluated the effects of an intensive 9-week numeracy and arithmetic programme for second-grade students at risk for early MDs. The focus of the programme was students' foundational understanding of numbers and mathematical concepts and procedural fluency with arithmetic tasks. Sample: A total of 753 first-grade students from 21 schools in Sweden were screened for low achievement in number knowledge and arithmetic. Methods: Students considered at risk for MDs ([less than or equal to] 25 percentile on two consecutive first-grade mathematics screenings) were individually randomized to an intervention group (n = 32) or control group (n = 30) at the beginning of second grade (7-8 years old). Trained teachers administered the one-to-one, explicit programme to intervention group students in elementary school settings. The intervention group received numeracy instruction emphasizing foundational mathematics concepts and procedures. Controls received teaching as usual with potential special education support provided by their schools. Results: The intervention group demonstrated significantly greater improvements in conceptual knowledge, arithmetic calculations and problem-solving compared to the control group, with medium size effects observed. Conclusions: A supplemental and intensive programme, with explicit instruction emphasizing numeracy, substantially improved knowledge and skills essential for arithmetic learning. Instruction in conceptual number knowledge and procedures also shows a significant impact on basic arithmetic problem-solving.