diff --git a/src/ParallelTestRunner.jl b/src/ParallelTestRunner.jl
index ae91180..5fc6092 100644
--- a/src/ParallelTestRunner.jl
+++ b/src/ParallelTestRunner.jl
@@ -360,7 +360,9 @@ Determine default number of parallel jobs.
 function default_njobs(; cpu_threads = Sys.CPU_THREADS, free_memory = available_memory())
     jobs = cpu_threads
     memory_jobs = Int64(free_memory) ÷ (2 * 2^30)
-    return max(1, min(jobs, memory_jobs))
+    env_var_num_jobs = tryparse(Int, get(ENV, "PARALLELTESTRUNNER_NUM_JOBS", ""))
+    # `PARALLELTESTRUNNER_NUM_JOBS` would take precedence over the default calculation.
+    return something(env_var_num_jobs, max(1, min(jobs, memory_jobs)))
 end
 
 # Historical test duration database
diff --git a/test/runtests.jl b/test/runtests.jl
index 6c83431..6bed69a 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -223,4 +223,11 @@ end
     @test contains(str, "SUCCESS")
 end
 
+@testset "Number of jobs" begin
+    num_jobs = 1234
+    withenv("PARALLELTESTRUNNER_NUM_JOBS" => num_jobs) do
+        @test ParallelTestRunner.default_njobs() == num_jobs
+    end
+end
+
 end