diff --git a/src/datajoint/diagram.py b/src/datajoint/diagram.py
@@ -1484,13 +1484,18 @@ def make_dot(self, theme=None):
 
         Notes
         -----
-        Layout direction is controlled via ``dj.config.display.diagram_direction``.
+        Layout direction is controlled via ``dj.config.display.diagram_direction``,
+        and the typeface of node and schema labels via
+        ``dj.config.display.diagram_fontname`` (default ``"Helvetica"``).
+        Graphviz sizes nodes with the face it finds on the rendering machine, so
+        a face that machine lacks changes the layout.
         Tables are grouped by schema, with the Python module name shown as the
         group label when available.
         """
         if not diagram_active:
             raise DataJointError("Install pygraphviz and pydot libraries to enable diagram visualization.")
         direction = self._connection._config.display.diagram_direction
+        fontname = self._connection._config.display.diagram_fontname
         graph = self._make_graph()
 
         # Apply collapse logic if needed
@@ -1630,7 +1635,7 @@ def _master_of(part_stripped):
             node.set_fontsize(props["fontsize"])
             node.set_fontcolor(props["fontcolor"])
             node.set_shape(props["shape"])
-            node.set_fontname("Helvetica")
+            node.set_fontname(fontname)
             node.set_fixedsize("shape" if props["fixed"] else False)
             node.set_width(props["size"])
             node.set_height(props["size"])
@@ -1742,7 +1747,7 @@ def _master_of(part_stripped):
                     style="rounded,dashed",
                     color=sc_color,
                     fontcolor=sc_fontcolor,
-                    fontname="Helvetica",  # schema label in a sans font, not Graphviz's Times default
+                    fontname=fontname,  # the node font, not Graphviz's Times default
                 )
                 node_by_name = {n.get_name().strip('"'): n for n in nodes}
                 # masters in this schema that have at least one part present
@@ -1840,10 +1845,11 @@ def make_mermaid(self) -> str:
         the tier palette is shared from ``_DIAGRAM_THEMES["light"]``; edge
         thickness encodes cardinality (thick = 1:1, thin = one-to-many),
         **not** primary-vs-secondary; renamed foreign keys are drawn in the
-        theme's amber. Three Graphviz features have no Mermaid equivalent and
-        are omitted: the top-right schema-label placement, the Helvetica font,
-        and the underline marking a dimension-introducing table. Dark mode is
-        left to the Mermaid host's theme rather than emitted inline.
+        theme's amber. Two Graphviz features have no Mermaid equivalent and
+        are omitted: the top-right schema-label placement and the underline
+        marking a dimension-introducing table. The typeface and dark mode are
+        left to the Mermaid host rather than emitted inline, so
+        ``display.diagram_fontname`` and ``display.diagram_theme`` do not apply.
 
         Examples
         --------

diff --git a/src/datajoint/settings.py b/src/datajoint/settings.py
@@ -77,6 +77,7 @@
     "loglevel": "DJ_LOG_LEVEL",
     "display.diagram_direction": "DJ_DIAGRAM_DIRECTION",
     "display.diagram_theme": "DJ_DIAGRAM_THEME",
+    "display.diagram_fontname": "DJ_DIAGRAM_FONTNAME",
 }
 
 Role = Enum("Role", "manual lookup imported computed job")
@@ -260,6 +261,16 @@ class DisplaySettings(BaseSettings):
             "mode; default), 'light', or 'dark' (dark background with adjusted palette)"
         ),
     )
+    diagram_fontname: str = Field(
+        default="Helvetica",
+        min_length=1,
+        validation_alias="DJ_DIAGRAM_FONTNAME",
+        description=(
+            "Typeface of node and schema labels in Graphviz diagrams (default 'Helvetica'). Graphviz sizes "
+            "nodes with the face it finds on the rendering machine, so a face that machine lacks changes "
+            "the layout. Set it only where the face is installed. Mermaid output names no typeface."
+        ),
+    )
 
 
 class StoresSettings(BaseSettings):

diff --git a/tests/integration/test_diagram_style.py b/tests/integration/test_diagram_style.py
@@ -9,6 +9,7 @@
 fails here; a layout tweak does not.
 """
 
+import re
 import time
 
 import pytest
@@ -111,6 +112,42 @@ def test_auto_theme_is_adaptive(schema_by_backend):
     assert "#e8f0e9" in svg and "#16281f" in svg, "auto theme must carry both light base and dark override colors"
 
 
+def _fontnames(dot_source):
+    """Every fontname value in a DOT source, in order of appearance."""
+    return [quoted or bare for quoted, bare in re.findall(r'fontname=(?:"([^"]*)"|([^\s,;\]]+))', dot_source)]
+
+
+def test_diagram_font_default_is_helvetica(schema_by_backend):
+    """Without the setting, every node and schema label stays in Helvetica (#1539)."""
+    if not dj.diagram.diagram_active:
+        pytest.skip("networkx/pydot not available")
+    ctx = _build(schema_by_backend)
+    assert dj.config.display.diagram_fontname == "Helvetica"
+    fonts = _fontnames(dj.Diagram(schema_by_backend, context=ctx).make_dot().to_string())
+    # six table nodes (Subject, Params, Session, Session.Note, Scan, Analysis) and one schema cluster
+    assert fonts == ["Helvetica"] * len(fonts) and len(fonts) >= 7
+
+
+def test_diagram_font_setting(schema_by_backend):
+    """``display.diagram_fontname`` sets the face of every node and schema label (#1539)."""
+    if not dj.diagram.diagram_active:
+        pytest.skip("networkx/pydot not available")
+    ctx = _build(schema_by_backend)
+    diagram = dj.Diagram(schema_by_backend, context=ctx)
+    default_count = len(_fontnames(diagram.make_dot().to_string()))
+    default_mermaid = diagram.make_mermaid()
+    with dj.config.override(display__diagram_fontname="Roboto Slab"):
+        dot_source = diagram.make_dot().to_string()
+        svg = diagram.svg_string()
+        mermaid = diagram.make_mermaid()
+    fonts = _fontnames(dot_source)
+    assert fonts == ["Roboto Slab"] * default_count
+    assert "Helvetica" not in dot_source
+    assert 'font-family="Roboto Slab' in svg
+    assert mermaid == default_mermaid  # Mermaid output leaves the typeface to its host
+    assert dj.config.display.diagram_fontname == "Helvetica"  # restored after the override
+
+
 def test_mermaid_matches_modernized_notation(schema_by_backend):
     """`make_mermaid` speaks the same notation as the Graphviz renderer:
     shared palette, cardinality-weighted edges, amber renamed FKs, and

diff --git a/tests/unit/test_settings.py b/tests/unit/test_settings.py
@@ -730,6 +730,27 @@ def test_display_limit(self):
         finally:
             dj.config.display.limit = original
 
+    def test_diagram_fontname_default(self, monkeypatch):
+        """Diagram labels default to Helvetica."""
+        from datajoint.settings import DisplaySettings
+
+        monkeypatch.delenv("DJ_DIAGRAM_FONTNAME", raising=False)
+        assert DisplaySettings().diagram_fontname == "Helvetica"
+
+    def test_diagram_fontname_from_env(self, monkeypatch):
+        """DJ_DIAGRAM_FONTNAME sets display.diagram_fontname, and outranks a config file value."""
+        from datajoint.settings import ENV_VAR_MAPPING, DisplaySettings
+
+        monkeypatch.setenv("DJ_DIAGRAM_FONTNAME", "Roboto Slab")
+        assert DisplaySettings().diagram_fontname == "Roboto Slab"
+        assert ENV_VAR_MAPPING["display.diagram_fontname"] == "DJ_DIAGRAM_FONTNAME"
+
+    def test_diagram_fontname_rejects_empty(self):
+        """An empty name would let Graphviz fall back to Times."""
+        with pytest.raises(ValidationError):
+            dj.config.display.diagram_fontname = ""
+        assert dj.config.display.diagram_fontname == "Helvetica"
+
 
 class TestCachePaths:
     """Test cache path settings."""