diff --git a/frontend/src/lib/ontology/ontology-loader.ts b/frontend/src/lib/ontology/ontology-loader.ts
index 30cc12ad5a..eb44db458f 100644
--- a/frontend/src/lib/ontology/ontology-loader.ts
+++ b/frontend/src/lib/ontology/ontology-loader.ts
@@ -623,8 +623,9 @@ function parseTurtleOntology(content: string): ParsedOntology {
   let blankNodeDepth = 0;  // Track depth of blank node blocks to skip
   let inMultiLineString = false;  // Track if we're inside a multi-line triple-quoted string
   let multiLineQuoteChar = '';  // The quote character(s) for the multi-line string
+  let baseUri: string | null = null;  // @base directive for relative URI resolution
   
-  // First pass: extract prefixes
+  // First pass: extract prefixes and @base
   for (const line of lines) {
     const trimmed = line.trim();
     if (trimmed.startsWith('@prefix') || trimmed.startsWith('PREFIX')) {
@@ -632,9 +633,58 @@ function parseTurtleOntology(content: string): ParsedOntology {
       if (match) {
         prefixes[match[1] || ''] = match[2];
       }
+    } else if (trimmed.startsWith('@base') || trimmed.startsWith('BASE')) {
+      // Extract @base directive: @base <http://example.org/> or BASE <http://example.org/>
+      const match = trimmed.match(/@?base\s+<([^>]+)>/i);
+      if (match) {
+        baseUri = match[1];
+      }
     }
   }
   
+  // Helper to expand URIs with base URI support for relative URIs
+  const expand = (uri: string): string => {
+    if (!uri) return uri;
+    
+    // Remove angle brackets if present
+    let cleaned = uri.trim().replace(/^</, '').replace(/>$/, '');
+    
+    // Handle empty relative URI <> - refers to the base URI itself
+    if (cleaned === '' && baseUri) {
+      return baseUri;
+    }
+    
+    // Handle relative URIs starting with # (like #Address)
+    // These resolve against the base URI
+    if (cleaned.startsWith('#') && baseUri) {
+      return baseUri + cleaned;
+    }
+    
+    // Check if it's already a full URI
+    if (cleaned.startsWith('http://') || cleaned.startsWith('https://')) {
+      return cleaned;
+    }
+    
+    // Handle prefixed URIs (like vcard:Address, owl:Class)
+    const colonIndex = cleaned.indexOf(':');
+    if (colonIndex > 0) {
+      const prefix = cleaned.substring(0, colonIndex);
+      const localName = cleaned.substring(colonIndex + 1);
+      
+      if (prefixes[prefix]) {
+        return prefixes[prefix] + localName;
+      }
+    }
+    
+    // Handle unprefixed URIs with default prefix (empty prefix ":")
+    // e.g., ":Address" with PREFIX : <http://example.org/#>
+    if (cleaned.startsWith(':') && prefixes['']) {
+      return prefixes[''] + cleaned.substring(1);
+    }
+    
+    return cleaned;
+  };
+  
   // Second pass: parse triples
   let lineNum = 0;
   for (const line of lines) {
@@ -734,16 +784,16 @@ function parseTurtleOntology(content: string): ParsedOntology {
     if (!trimmed.startsWith(';') && !trimmed.startsWith(',') && !isIndentedLine) {
       // Process previous subject if exists
       if (currentSubject && currentTriples.length > 0) {
-        processSubject(currentSubject, currentTriples, prefixes, classes, properties, individuals);
+        processSubject(currentSubject, currentTriples, prefixes, classes, properties, individuals, baseUri);
       }
       
       // Start new subject
       const parts = splitTurtleLine(trimmed, prefixes);
       if (parts.length >= 3) {
         // Full triple on one line: subject predicate object(s)
-        currentSubject = expandUri(parts[0], prefixes);
+        currentSubject = expand(parts[0]);
         // Handle 'a' shorthand for rdf:type
-        const predicate = parts[1] === 'a' ? NAMESPACES.rdf + 'type' : expandUri(parts[1], prefixes);
+        const predicate = parts[1] === 'a' ? NAMESPACES.rdf + 'type' : expand(parts[1]);
         lastPredicate = predicate;  // Track for comma continuations
         // Handle comma-separated values (e.g., "Subject a Class1, Class2")
         currentTriples = [];
@@ -752,7 +802,7 @@ function parseTurtleOntology(content: string): ParsedOntology {
         }
       } else if (parts.length === 1 && !trimmed.endsWith('.')) {
         // Subject alone on a line (DCAT3 style): dcat:Catalog
-        currentSubject = expandUri(parts[0], prefixes);
+        currentSubject = expand(parts[0]);
         currentTriples = [];
         lastPredicate = null;
       }
@@ -761,7 +811,7 @@ function parseTurtleOntology(content: string): ParsedOntology {
       const parts = splitTurtleLine(trimmed.substring(1).trim(), prefixes);
       if (parts.length >= 2) {
         // Handle 'a' shorthand for rdf:type
-        const predicate = parts[0] === 'a' ? NAMESPACES.rdf + 'type' : expandUri(parts[0], prefixes);
+        const predicate = parts[0] === 'a' ? NAMESPACES.rdf + 'type' : expand(parts[0]);
         lastPredicate = predicate;  // Track for comma continuations
         // Handle comma-separated values
         for (let i = 1; i < parts.length; i++) {
@@ -774,7 +824,7 @@ function parseTurtleOntology(content: string): ParsedOntology {
       const parts = splitTurtleLine(trimmed, prefixes);
       if (parts.length >= 2) {
         // Handle 'a' shorthand for rdf:type
-        const predicate = parts[0] === 'a' ? NAMESPACES.rdf + 'type' : expandUri(parts[0], prefixes);
+        const predicate = parts[0] === 'a' ? NAMESPACES.rdf + 'type' : expand(parts[0]);
         lastPredicate = predicate;  // Track for comma continuations
         // Handle comma-separated values (e.g., "a rdfs:Class, owl:Class")
         // Each part after predicate is a separate object
@@ -894,7 +944,8 @@ function processSubject(
   prefixes: Record<string, string>,
   classes: Map<string, OntologyClass>,
   properties: Map<string, OntologyProperty>,
-  individuals: Map<string, OntologyIndividual>
+  individuals: Map<string, OntologyIndividual>,
+  baseUri?: string | null
 ): void {
   const types: string[] = [];
   const labels: LangString[] = [];
@@ -915,7 +966,7 @@ function processSubject(
   
   for (const triple of triples) {
     const { predicate, object } = triple;
-    const expandedObject = expandUri(object, prefixes);
+    const expandedObject = expandUri(object, prefixes, baseUri);
     
     if (predicate === NAMESPACES.rdf + 'type') {
       types.push(expandedObject);
@@ -1506,19 +1557,33 @@ function parseJsonLdOntology(content: string): ParsedOntology {
 
 /**
  * Expand prefixed URI to full URI
+ * @param uri - The URI to expand (can be prefixed like "owl:Class", relative like "#Address", or full)
+ * @param prefixes - Map of prefix to namespace
+ * @param baseUri - Optional base URI for resolving relative URIs (from @base directive)
  */
-export function expandUri(uri: string, prefixes: Record<string, string>): string {
+export function expandUri(uri: string, prefixes: Record<string, string>, baseUri?: string | null): string {
   if (!uri) return uri;
   
   // Remove angle brackets if present
   let cleaned = uri.trim().replace(/^</, '').replace(/>$/, '');
   
+  // Handle empty relative URI <> - refers to the base URI itself
+  if (cleaned === '' && baseUri) {
+    return baseUri;
+  }
+  
+  // Handle relative URIs starting with # (like #Address)
+  // These resolve against the base URI
+  if (cleaned.startsWith('#') && baseUri) {
+    return baseUri + cleaned;
+  }
+  
   // Check if it's already a full URI
   if (cleaned.startsWith('http://') || cleaned.startsWith('https://')) {
     return cleaned;
   }
   
-  // Handle prefixed URIs
+  // Handle prefixed URIs (like vcard:Address, owl:Class)
   const colonIndex = cleaned.indexOf(':');
   if (colonIndex > 0) {
     const prefix = cleaned.substring(0, colonIndex);
@@ -1529,6 +1594,12 @@ export function expandUri(uri: string, prefixes: Record<string, string>): string
     }
   }
   
+  // Handle unprefixed URIs with default prefix (empty prefix ":")
+  // e.g., ":Address" with PREFIX : <http://example.org/#>
+  if (cleaned.startsWith(':') && prefixes['']) {
+    return prefixes[''] + cleaned.substring(1);
+  }
+  
   return cleaned;
 }