diff --git a/reframe/core/schedulers/slurm.py b/reframe/core/schedulers/slurm.py
index 773ef893d9..a0c9f5620a 100644
--- a/reframe/core/schedulers/slurm.py
+++ b/reframe/core/schedulers/slurm.py
@@ -161,6 +161,15 @@ def _get_all_nodes(self):
         node_descriptions = completed.stdout.splitlines()
         return {SlurmNode(descr) for descr in node_descriptions}
 
+    def _get_default_partition(self):
+        completed = _run_strict('scontrol -a show -o partitions')
+        partition_match = re.search(r'PartitionName=(?P<partition>\S+)\s+'
+                                    r'.*Default=YES.*', completed.stdout)
+        if partition_match:
+            return partition_match.group('partition')
+
+        return None
+
     def get_partition_nodes(self):
         nodes = self._get_all_nodes()
         return self.filter_nodes(nodes, self.sched_access)
@@ -187,10 +196,14 @@ def filter_nodes(self, nodes, options):
 
         if partitions:
             partitions = set(partitions.strip().split(','))
-            nodes = {n for n in nodes if n.partitions >= partitions}
-            getlogger().debug(
-                'flex_alloc_tasks: filtering nodes by partition(s) %s: '
-                'available nodes now: %s' % (partitions, len(nodes)))
+        else:
+            default_partition = self._get_default_partition()
+            partitions = {default_partition} if default_partition else set()
+
+        nodes = {n for n in nodes if n.partitions >= partitions}
+        getlogger().debug(
+            'flex_alloc_tasks: filtering nodes by partition(s) %s: '
+            'available nodes now: %s' % (partitions, len(nodes)))
 
         if constraints:
             constraints = set(constraints.strip().split(','))
diff --git a/unittests/test_schedulers.py b/unittests/test_schedulers.py
index d8bdc982fb..cbab7176e6 100644
--- a/unittests/test_schedulers.py
+++ b/unittests/test_schedulers.py
@@ -396,6 +396,9 @@ def test_guess_num_tasks(self):
         # monkey patch `get_partition_nodes()` to simulate extraction of
         # slurm nodes through the use of `scontrol show`
         self.testjob.get_partition_nodes = lambda: set()
+        # monkey patch `_get_default_partition()` to simulate extraction
+        # of the default partition through the use of `scontrol show`
+        self.testjob._get_default_partition = lambda: 'pdef'
         self.assertEqual(self.testjob.guess_num_tasks(), 0)
 
 
@@ -502,7 +505,7 @@ def create_dummy_nodes(obj):
                              'RealMemory=32220 AllocMem=0 FreeMem=10000 '
                              'Sockets=1 Boards=1 State=MAINT+DRAIN '
                              'ThreadsPerCore=2 TmpDisk=0 Weight=1 Owner=N/A '
-                             'MCS_label=N/A Partitions=p1,p2 '
+                             'MCS_label=N/A Partitions=p1,p2,pdef '
                              'BootTime=01 Jan 2018 '
                              'SlurmdStartTime=01 Jan 2018 '
                              'CfgTRES=cpu=24,mem=32220M '
@@ -520,7 +523,7 @@ def create_dummy_nodes(obj):
                              'RealMemory=32220 AllocMem=0 FreeMem=10000 '
                              'Sockets=1 Boards=1 State=MAINT+DRAIN '
                              'ThreadsPerCore=2 TmpDisk=0 Weight=1 Owner=N/A '
-                             'MCS_label=N/A Partitions=p2,p3'
+                             'MCS_label=N/A Partitions=p2,p3,pdef '
                              'BootTime=01 Jan 2018 '
                              'SlurmdStartTime=01 Jan 2018 '
                              'CfgTRES=cpu=24,mem=32220M '
@@ -538,7 +541,7 @@ def create_dummy_nodes(obj):
                              'RealMemory=32220 AllocMem=0 FreeMem=10000 '
                              'Sockets=1 Boards=1 State=IDLE '
                              'ThreadsPerCore=2 TmpDisk=0 Weight=1 Owner=N/A '
-                             'MCS_label=N/A Partitions=p1,p3 '
+                             'MCS_label=N/A Partitions=p1,p3,pdef '
                              'BootTime=01 Jan 2018 '
                              'SlurmdStartTime=01 Jan 2018 '
                              'CfgTRES=cpu=24,mem=32220M '
@@ -556,7 +559,7 @@ def create_dummy_nodes(obj):
                              'RealMemory=32220 AllocMem=0 FreeMem=10000 '
                              'Sockets=1 Boards=1 State=IDLE '
                              'ThreadsPerCore=2 TmpDisk=0 Weight=1 Owner=N/A '
-                             'MCS_label=N/A Partitions=p1,p3 '
+                             'MCS_label=N/A Partitions=p1,p3,pdef '
                              'BootTime=01 Jan 2018 '
                              'SlurmdStartTime=01 Jan 2018 '
                              'CfgTRES=cpu=24,mem=32220M '
@@ -589,6 +592,9 @@ def setUp(self):
         # monkey patch `_get_all_nodes` to simulate extraction of
         # slurm nodes through the use of `scontrol show`
         self.testjob._get_all_nodes = self.create_dummy_nodes
+        # monkey patch `_get_default_partition` to simulate extraction
+        # of the default partition
+        self.testjob._get_default_partition = lambda: 'pdef'
         self.testjob._sched_flex_alloc_tasks = 'all'
         self.testjob._num_tasks_per_node = 4
         self.testjob._num_tasks = 0