diff --git a/docs/syntax/entityRelationshipDiagram.md b/docs/syntax/entityRelationshipDiagram.md
index ea3b8c00541..a4cfaf751bb 100644
--- a/docs/syntax/entityRelationshipDiagram.md
+++ b/docs/syntax/entityRelationshipDiagram.md
@@ -90,7 +90,7 @@ Mermaid syntax for ER diagrams is compatible with PlantUML, with an extension to
 
 Where:
 
-- `first-entity` is the name of an entity. Names must begin with an alphabetic character and may also contain digits, hyphens, and underscores.
+- `first-entity` is the name of an entity. Names must begin with an alphabetic character or an underscore (from v\<MERMAID_RELEASE_VERSION>+), and may also contain digits, hyphens, and underscores.
 - `relationship` describes the way that both entities inter-relate. See below.
 - `second-entity` is the name of the other entity.
 - `relationship-label` describes the relationship from the perspective of the first entity.
diff --git a/packages/mermaid/src/diagrams/er/parser/erDiagram.jison b/packages/mermaid/src/diagrams/er/parser/erDiagram.jison
index 4abc6557cc5..f3913460773 100644
--- a/packages/mermaid/src/diagrams/er/parser/erDiagram.jison
+++ b/packages/mermaid/src/diagrams/er/parser/erDiagram.jison
@@ -66,7 +66,7 @@ o\{                             return 'ZERO_OR_MORE';
 "optionally to"                 return 'NON_IDENTIFYING';
 \.\-                            return 'NON_IDENTIFYING';
 \-\.                            return 'NON_IDENTIFYING';
-[A-Za-z][A-Za-z0-9\-_]*         return 'ALPHANUM';
+[A-Za-z_][A-Za-z0-9\-_]*        return 'ALPHANUM';
 .                               return yytext[0];
 <<EOF>>                         return 'EOF';
 
diff --git a/packages/mermaid/src/diagrams/er/parser/erDiagram.spec.js b/packages/mermaid/src/diagrams/er/parser/erDiagram.spec.js
index 6b0865a2d47..e9fe945cfcf 100644
--- a/packages/mermaid/src/diagrams/er/parser/erDiagram.spec.js
+++ b/packages/mermaid/src/diagrams/er/parser/erDiagram.spec.js
@@ -30,10 +30,10 @@ describe('when parsing ER diagram it...', function () {
         expect(entities.hasOwnProperty(name)).toBe(false);
       }).toThrow();
     });
-    describe('has non A-Za-z0-9_- chars', function () {
+    describe('has non A-Za-z0-9- chars', function () {
       // these were entered using the Mac keyboard utility.
       const chars =
-        "~ ` ! @ # $ ^ & * ( ) - _ = + [ ] { } | / ; : ' . ? ¡ ⁄ ™ € £ ‹ ¢ › ∞ fi § ‡ • ° ª · º ‚ ≠ ± œ Œ ∑ „ ® † ˇ ¥ Á ¨ ˆ ˆ Ø π ∏ “ « » å Å ß Í ∂ Î ƒ Ï © ˙ Ó ∆ Ô ˚  ¬ Ò … Ú æ Æ Ω ¸ ≈ π ˛ ç Ç √ ◊ ∫ ı ˜ µ  ≤ ¯ ≥ ˘ ÷ ¿";
+        "~ ` ! @ # $ ^ & * ( ) - = + [ ] { } | / ; : ' . ? ¡ ⁄ ™ € £ ‹ ¢ › ∞ fi § ‡ • ° ª · º ‚ ≠ ± œ Œ ∑ „ ® † ˇ ¥ Á ¨ ˆ ˆ Ø π ∏ “ « » å Å ß Í ∂ Î ƒ Ï © ˙ Ó ∆ Ô ˚  ¬ Ò … Ú æ Æ Ω ¸ ≈ π ˛ ç Ç √ ◊ ∫ ı ˜ µ  ≤ ¯ ≥ ˘ ÷ ¿";
       const allowed = chars.split(' ');
 
       allowed.forEach((allowedChar) => {
@@ -170,6 +170,13 @@ describe('when parsing ER diagram it...', function () {
       expect(entities[firstEntity].alias).toBe(alias);
       expect(entities[secondEntity].alias).toBeUndefined();
     });
+
+    it('can start with an underscore', function () {
+      const entity = '_foo';
+      erDiagram.parser.parse(`erDiagram\n${entity}\n`);
+      const entities = erDb.getEntities();
+      expect(entities.hasOwnProperty(entity)).toBe(true);
+    });
   });
 
   describe('attribute name', () => {
diff --git a/packages/mermaid/src/docs/syntax/entityRelationshipDiagram.md b/packages/mermaid/src/docs/syntax/entityRelationshipDiagram.md
index f2a5d5481fa..93beed16262 100644
--- a/packages/mermaid/src/docs/syntax/entityRelationshipDiagram.md
+++ b/packages/mermaid/src/docs/syntax/entityRelationshipDiagram.md
@@ -56,7 +56,7 @@ Mermaid syntax for ER diagrams is compatible with PlantUML, with an extension to
 
 Where:
 
-- `first-entity` is the name of an entity. Names must begin with an alphabetic character and may also contain digits, hyphens, and underscores.
+- `first-entity` is the name of an entity. Names must begin with an alphabetic character or an underscore (from v<MERMAID_RELEASE_VERSION>+), and may also contain digits, hyphens, and underscores.
 - `relationship` describes the way that both entities inter-relate. See below.
 - `second-entity` is the name of the other entity.
 - `relationship-label` describes the relationship from the perspective of the first entity.