Dysregulation of the immune system is one potential mechanism by which acute stress may contribute to downstream disease etiology and psychopathology. Here, we tested the role of β-adrenergic signaling as a mediator of acute stress-induced changes in immune cell gene expression. In a randomized, double-blind, and placebo-controlled trial, 90 healthy young adults (44% female) received a single 40 mg dose of the β-blocker propranolol (n = 43) or a placebo (n = 47) and then completed the Trier Social Stress Test (TSST). Pre- and post-stress blood samples were assayed for prespecified sets of pro-inflammatory and antiviral/antibody gene transcripts. Analyses revealed increased expression of both inflammatory and antiviral/antibody-related genes in response to the TSST, and these effects were blocked by pre-treatment with propranolol. Bioinformatics identified natural killer cells and dendritic cells as the primary cellular context for transcriptional upregulation, and monocytes as the primary cellular carrier of genes downregulated by the TSST. These effects were in part explained by acute changes in circulating cell types. Results suggest that acute psychosocial stress can induce an “acute defense” molecular phenotype via β-adrenergic signaling that involves mobilization of natural killer cells and dendritic cells at the expense of monocytes. This may represent an adaptive response to the risk of acute injury. These findings offer some of the first evidence in humans that β-blockade attenuates psychosocial stress-induced increases in inflammatory gene expression, offering new insights into the molecular and immunologic pathways by which stress may confer risks to health and well-being.