diff --git a/dask_jobqueue/lsf.py b/dask_jobqueue/lsf.py
index d84afbf7..d079ac55 100644
--- a/dask_jobqueue/lsf.py
+++ b/dask_jobqueue/lsf.py
@@ -87,7 +87,7 @@ def __init__(
             memory_string = lsf_format_bytes_ceil(mem, lsf_units=lsf_units)
             header_lines.append("#BSUB -M %s" % memory_string)
         if walltime is not None:
-            header_lines.append("#BSUB -W %s" % walltime)
+            header_lines.append("#BSUB -W %s" % lsf_format_walltime(walltime))
 
         # Skip requested header directives
         header_lines = list(
@@ -136,6 +136,37 @@ def lsf_format_bytes_ceil(n, lsf_units="mb"):
     return "%d" % math.ceil(n / (1000 ** converter[lsf_units]))
 
 
+def lsf_format_walltime(walltime):
+    """Format a walltime as ``[hour:]minute``, which is what LSF requires
+
+    The ``HH:MM:SS`` format accepted by the other job schedulers is converted,
+    rounding any leftover seconds up to the next minute so that the job is
+    never given less time than requested. Any other value is returned
+    unchanged.
+
+    Parameters
+    ----------
+    walltime: str
+        Walltime of the job
+
+    Examples
+    --------
+    >>> lsf_format_walltime("01:30:00")
+    '01:30'
+    >>> lsf_format_walltime("00:00:30")
+    '00:01'
+    >>> lsf_format_walltime("01:30")
+    '01:30'
+    """
+    parts = str(walltime).split(":")
+    if len(parts) != 3 or not all(part.isdigit() for part in parts):
+        return walltime
+    hours, minutes, seconds = (int(part) for part in parts)
+    total_minutes = hours * 60 + minutes + math.ceil(seconds / 60)
+    hours, minutes = divmod(total_minutes, 60)
+    return "%02d:%02d" % (hours, minutes)
+
+
 def lsf_detect_units():
     """Try to autodetect the unit scaling on an LSF system"""
     # Search for automatically, Using docs from LSF 9.1.3 for search/defaults
@@ -188,7 +219,8 @@ class LSFCluster(JobQueueCluster):
     mem : int
         Request memory in bytes. Passed to `#BSUB -M` option.
     walltime : str
-        Walltime for each worker job in HH:MM. Passed to `#BSUB -W` option.
+        Walltime for each worker job in HH:MM. A walltime given as HH:MM:SS is
+        converted to HH:MM, rounding up. Passed to `#BSUB -W` option.
     {cluster}
     job_extra : list
         Deprecated: use ``job_extra_directives`` instead. This parameter will be removed in a future version.
diff --git a/dask_jobqueue/tests/test_lsf.py b/dask_jobqueue/tests/test_lsf.py
index 5f8c7435..9680d81f 100644
--- a/dask_jobqueue/tests/test_lsf.py
+++ b/dask_jobqueue/tests/test_lsf.py
@@ -79,6 +79,27 @@ def test_header():
         assert "#BSUB -P" not in cluster.job_header
 
 
+@pytest.mark.parametrize(
+    "walltime, expected",
+    [
+        ("01:00:00", "01:00"),
+        ("00:30:00", "00:30"),
+        ("2:15:00", "02:15"),
+        ("00:00:30", "00:01"),
+        ("01:59:01", "02:00"),
+        ("48:00:00", "48:00"),
+        ("01:00", "01:00"),
+        ("1:00", "1:00"),
+        ("90", "90"),
+    ],
+)
+def test_walltime_format(walltime, expected):
+    assert lsf.lsf_format_walltime(walltime) == expected
+
+    with LSFCluster(walltime=walltime, cores=1, memory="1GB") as cluster:
+        assert "#BSUB -W %s" % expected in cluster.job_header.splitlines()
+
+
 def test_job_script():
     with LSFCluster(walltime="00:02", processes=4, cores=8, memory="28GB") as cluster:
         job_script = cluster.job_script()
diff --git a/docs/source/changelog.rst b/docs/source/changelog.rst
index b6661ffa..b38dc0f6 100644
--- a/docs/source/changelog.rst
+++ b/docs/source/changelog.rst
@@ -4,6 +4,8 @@ Changelog
 Development version
 -------------------
 
+- Fixed ``LSFCluster`` generating an invalid ``#BSUB -W`` directive for a walltime in ``HH:MM:SS`` format (:issue:`703`)
+
 0.9.0 / 2024-08-22
 ------------------