This study examined the role of fidelity in the reading outcomes within a Tier 2 intervention implemented by Spanish kindergarten and first grade school teachers. For this purpose, differences in reading performance growth were analyzed among at-risk students who received a Tier 2 intervention with high fidelity, at-risk students who received the same intervention with medium fidelity, and an at-risk control group that did not receive a Tier 2 intervention. Implementation fidelity was analyzed using direct observations and self-reports. All students were assessed three times during the academic year. Hierarchical linear modeling analyses were conducted to explore differences in growth rate. The at-risk children in the intervention condition showed a higher growth compared to at-risk children in the control condition, and specifically when the intervention was delivered with a high degree of fidelity.