diff --git a/lib/rules/multiline-comment-style.js b/lib/rules/multiline-comment-style.js
index cccc841cd867..8cf497b786e1 100644
--- a/lib/rules/multiline-comment-style.js
+++ b/lib/rules/multiline-comment-style.js
@@ -18,13 +18,20 @@ module.exports = {
             recommended: false
         },
         fixable: "whitespace",
-        schema: [{ enum: ["starred-block", "separate-lines"] }]
+        schema: [{ enum: ["starred-block", "separate-lines", "bare-block"] }]
     },
 
     create(context) {
         const sourceCode = context.getSourceCode();
         const option = context.options[0] || "starred-block";
 
+        const EXPECTED_BLOCK_ERROR = "Expected a block comment instead of consecutive line comments.";
+        const START_NEWLINE_ERROR = "Expected a linebreak after '/*'.";
+        const END_NEWLINE_ERROR = "Expected a linebreak before '*/'.";
+        const MISSING_STAR_ERROR = "Expected a '*' at the start of this line.";
+        const ALIGNMENT_ERROR = "Expected this line to be aligned with the start of the comment.";
+        const EXPECTED_LINES_ERROR = "Expected multiple line comments instead of a block comment.";
+
         //----------------------------------------------------------------------
         // Helpers
         //----------------------------------------------------------------------
@@ -56,6 +63,32 @@ module.exports = {
             return `\n${starredLines.join("\n")}\n${initialOffset} `;
         }
 
+        /**
+         * Converts a comment into separate-line form
+         * @param {Token} firstComment The first comment of the group being converted
+         * @param {string[]} commentLinesList A list of lines to appear in the new starred-block comment
+         * @returns {string} A representation of the comment value in separate-line form
+         */
+        function convertToSeparateLines(firstComment, commentLinesList) {
+            const initialOffset = sourceCode.text.slice(firstComment.range[0] - firstComment.loc.start.column, firstComment.range[0]);
+            const separateLines = commentLinesList.map(line => `// ${line.trim()}`);
+
+            return separateLines.join(`\n${initialOffset}`);
+        }
+
+        /**
+         * Converts a comment into bare-block form
+         * @param {Token} firstComment The first comment of the group being converted
+         * @param {string[]} commentLinesList A list of lines to appear in the new starred-block comment
+         * @returns {string} A representation of the comment value in bare-block form
+         */
+        function convertToBlock(firstComment, commentLinesList) {
+            const initialOffset = sourceCode.text.slice(firstComment.range[0] - firstComment.loc.start.column, firstComment.range[0]);
+            const blockLines = commentLinesList.map(line => line.trim());
+
+            return `/* ${blockLines.join(`\n${initialOffset}`)} */`;
+        }
+
         /**
          * Each method checks a group of comments to see if it's valid according to the given option.
          * @param {Token[]} commentGroup A list of comments that appear together. This will either contain a single
@@ -76,7 +109,7 @@ module.exports = {
                             start: commentGroup[0].loc.start,
                             end: commentGroup[commentGroup.length - 1].loc.end
                         },
-                        message: "Expected a block comment instead of consecutive line comments.",
+                        message: EXPECTED_BLOCK_ERROR,
                         fix(fixer) {
                             return fixer.replaceTextRange(
                                 [commentGroup[0].range[0], commentGroup[commentGroup.length - 1].range[1]],
@@ -97,7 +130,7 @@ module.exports = {
                                 start: block.loc.start,
                                 end: { line: block.loc.start.line, column: block.loc.start.column + 2 }
                             },
-                            message: "Expected a linebreak after '/*'.",
+                            message: START_NEWLINE_ERROR,
                             fix: fixer => fixer.insertTextAfterRange([start, start + 2], `\n${expectedLinePrefix}`)
                         });
                     }
@@ -108,7 +141,7 @@ module.exports = {
                                 start: { line: block.loc.end.line, column: block.loc.end.column - 2 },
                                 end: block.loc.end
                             },
-                            message: "Expected a linebreak before '*/'.",
+                            message: END_NEWLINE_ERROR,
                             fix: fixer => fixer.replaceTextRange([block.range[1] - 2, block.range[1]], `\n${expectedLinePrefix}/`)
                         });
                     }
@@ -124,8 +157,8 @@ module.exports = {
                                     end: { line: lineNumber, column: sourceCode.lines[lineNumber - 1].length }
                                 },
                                 message: /^\s*\*/.test(lineText)
-                                    ? "Expected this line to be aligned with the start of the comment."
-                                    : "Expected a '*' at the start of this line.",
+                                    ? ALIGNMENT_ERROR
+                                    : MISSING_STAR_ERROR,
                                 fix(fixer) {
 
                                     // TODO: Make this more readable, possibly by splitting it into two separate cases.
@@ -140,9 +173,44 @@ module.exports = {
                     }
                 }
             },
-            "separate-lines"() {
+            "separate-lines"(commentGroup) {
 
                 // TODO
+                if (commentGroup[0].type === "Block") {
+                    const block = commentGroup[0];
+
+                    context.report({
+                        loc: {
+                            start: block.loc.start,
+                            end: { line: block.loc.start.line, column: block.loc.start.column + 2 }
+                        },
+                        message: EXPECTED_LINES_ERROR,
+                        fix(fixer) {
+                            const commentLines = getCommentLines(commentGroup).filter(line => line.trim());
+
+                            return fixer.replaceTextRange([block.range[0], block.range[1]], convertToSeparateLines(block, commentLines));
+                        }
+                    });
+                }
+            },
+            "bare-block"(commentGroup) {
+                let commentLines = getCommentLines(commentGroup);
+
+                if (commentGroup[0].type === "Line" && commentLines.length > 1) {
+
+                    context.report({
+                        loc: {
+                            start: commentGroup[0].loc.start,
+                            end: commentGroup[commentGroup.length - 1].loc.end
+                        },
+                        message: EXPECTED_BLOCK_ERROR,
+                        fix(fixer) {
+                            commentLines = commentLines.filter(line => line.trim());
+
+                            return fixer.replaceTextRange([commentGroup[0].range[0], commentGroup[commentGroup.length - 1].range[1]], convertToBlock(commentGroup[0], commentLines));
+                        }
+                    });
+                }
             }
         };
 
diff --git a/tests/lib/rules/multiline-comment-style.js b/tests/lib/rules/multiline-comment-style.js
index 70f6ea9f7b2d..c40d109e2290 100644
--- a/tests/lib/rules/multiline-comment-style.js
+++ b/tests/lib/rules/multiline-comment-style.js
@@ -339,6 +339,20 @@ ruleTester.run("multiline-comment-style", rule, {
             `,
             options: ["separate-lines"],
             errors: [{ message: EXPECTED_LINES_ERROR, line: 2 }]
+        },
+
+        // TODO: add "bare-block" option
+        {
+            code: `
+                // foo
+                // bar
+            `,
+            output: `
+                /* foo
+                bar */
+            `,
+            options: ["bare-block"],
+            errors: [{ message: EXPECTED_BLOCK_ERROR, line: 2 }]
         }
     ]
 });